spi: Add spi_bpw_to_bytes() helper and use it

This helper converts the given bits per word to bytes. The result
will always be power-of-two, e.g.,

    ===============    =================
    Input (in bits)    Output (in bytes)
    ===============    =================
            5                   1
            9                   2
            21                  4
            37                  8
    ===============    =================

It will return 0 for the 0 input.

There are a couple of cases in SPI that are using the same approach
and at least one more (in IIO) would benefit of it. Add a helper
for everyone.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250417152529.490582-2-andriy.shevchenko@linux.intel.com
Acked-by: Mukesh Kumar Savaliya <quic_msavaliy@quicinc.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: andy-shev

drivers/spi/spi.c

@@ -3802,7 +3802,7 @@ int spi_split_transfers_maxwords(struct spi_controller *ctlr,
 		size_t maxsize;
 		int ret;
 
-		maxsize = maxwords * roundup_pow_of_two(BITS_TO_BYTES(xfer->bits_per_word));
+		maxsize = maxwords * spi_bpw_to_bytes(xfer->bits_per_word);
 		if (xfer->len > maxsize) {
 			ret = __spi_split_transfer_maxsize(ctlr, msg, &xfer,
 							   maxsize);

include/linux/spi/spi.h

@@ -1325,6 +1325,32 @@ static inline bool spi_is_bpw_supported(struct spi_device *spi, u32 bpw)
 	return false;
 }
 
+/**
+ * spi_bpw_to_bytes - Covert bits per word to bytes
+ * @bpw: Bits per word
+ *
+ * This function converts the given @bpw to bytes. The result is always
+ * power-of-two, e.g.,
+ *
+ *  ===============    =================
+ *  Input (in bits)    Output (in bytes)
+ *  ===============    =================
+ *          5                   1
+ *          9                   2
+ *          21                  4
+ *          37                  8
+ *  ===============    =================
+ *
+ * It will return 0 for the 0 input.
+ *
+ * Returns:
+ * Bytes for the given @bpw.
+ */
+static inline u32 spi_bpw_to_bytes(u32 bpw)
+{
+	return roundup_pow_of_two(BITS_TO_BYTES(bpw));
+}
+
 /**
  * spi_controller_xfer_timeout - Compute a suitable timeout value
  * @ctlr: SPI device