nvmem: rmem: Fix return value of rmem_read()

reg_read() callback registered with nvmem core expects 0 on success and
a negative value on error but rmem_read() returns the number of bytes
read which is treated as an error at the nvmem core.

This does not break when rmem is accessed using sysfs via
bin_attr_nvmem_read()/write() but causes an error when accessed from
places like nvmem_access_with_keepouts(), etc.

Change to return 0 on success and error in case
memory_read_from_buffer() returns an error or -EIO if bytes read do not
match what was requested.

Fixes: 5a3fa75 ("nvmem: Add driver to expose reserved memory as nvmem")
Cc: stable@vger.kernel.org
Signed-off-by: Joy Chakraborty <joychakr@google.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20240628113704.13742-2-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Joy Chakraborty

drivers/nvmem/rmem.c

@@ -46,7 +46,10 @@ static int rmem_read(void *context, unsigned int offset,
 
 	memunmap(addr);
 
-	return count;
+	if (count < 0)
+		return count;
+
+	return count == bytes ? 0 : -EIO;
 }
 
 static int rmem_probe(struct platform_device *pdev)