spi: rzv2h-rspi: enable TX buffer empty interrupt

In preparation for implementing DMA support, enable the transmit buffer
empty interrupt, which is necessary for DMA to write more data to the
FIFO.

This does not affect the PIO mode as we do not even request the TX
interrupt line.

Signed-off-by: Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>
Link: https://patch.msgid.link/20251201134229.600817-9-cosmin-gabriel.tanislav.xa@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: Demon000

drivers/spi/spi-rzv2h-rspi.c

@@ -37,6 +37,7 @@
 /* Register SPCR */
 #define RSPI_SPCR_BPEN		BIT(31)
 #define RSPI_SPCR_MSTR		BIT(30)
+#define RSPI_SPCR_SPTIE		BIT(20)
 #define RSPI_SPCR_SPRIE		BIT(17)
 #define RSPI_SPCR_SCKASE	BIT(12)
 #define RSPI_SPCR_SPE		BIT(0)
@@ -474,6 +475,9 @@ static int rzv2h_rspi_prepare_message(struct spi_controller *ctlr,
 	/* SPI receive buffer full interrupt enable */
 	conf32 |= RSPI_SPCR_SPRIE;
 
+	/* SPI transmit buffer empty interrupt enable */
+	conf32 |= RSPI_SPCR_SPTIE;
+
 	/* Bypass synchronization circuit */
 	conf32 |= FIELD_PREP(RSPI_SPCR_BPEN, rspi->use_pclk);