drivers: spi: remove '&' when assigning `init_fn`

Remove address-of operator ('&') when assigning `init_fn`
function pointer in `DEVICE_DT_INST_DEFINE` macro.

This change aims to maintain consistency among the drivers in
`drivers/spi`, ensuring that all function pointer assignments
follow the same pattern.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>
diff --git a/drivers/spi/spi_mcux_flexio.c b/drivers/spi/spi_mcux_flexio.c
index 69bb996..1e00792 100644
--- a/drivers/spi/spi_mcux_flexio.c
+++ b/drivers/spi/spi_mcux_flexio.c
@@ -429,7 +429,7 @@
 		SPI_CONTEXT_CS_GPIOS_INITIALIZE(DT_DRV_INST(n), ctx)	\
 	};								\
 									\
-	DEVICE_DT_INST_DEFINE(n, &spi_mcux_init, NULL,			\
+	DEVICE_DT_INST_DEFINE(n, spi_mcux_init, NULL,			\
 				&spi_mcux_flexio_data_##n,		\
 				&spi_mcux_flexio_config_##n, POST_KERNEL, \
 				CONFIG_SPI_INIT_PRIORITY,		\