spi: xilinx_spi: Add missing prototype for xilinx_qspi_mem_exec_op

Add missing prototype to fix the below sparse warning
warning: no previous prototype for 'xilinx_qspi_mem_exec_op'
[-Wmissing-prototypes]

Signed-off-by: Algapally Santosh Sagar <santoshsagar.algapally@amd.com>
Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>
Link: https://lore.kernel.org/r/20230614090359.10809-2-ashok.reddy.soma@amd.com
Signed-off-by: Michal Simek <michal.simek@amd.com>
diff --git a/drivers/spi/xilinx_spi.c b/drivers/spi/xilinx_spi.c
index 33575fe..b58a3f6 100644
--- a/drivers/spi/xilinx_spi.c
+++ b/drivers/spi/xilinx_spi.c
@@ -363,8 +363,8 @@
 	return -EOPNOTSUPP;
 }
 
-bool xilinx_qspi_mem_exec_op(struct spi_slave *slave,
-			     const struct spi_mem_op *op)
+static bool xilinx_qspi_mem_exec_op(struct spi_slave *slave,
+				    const struct spi_mem_op *op)
 {
 	if (xilinx_qspi_check_buswidth(slave, op->cmd.buswidth))
 		return false;