net: fm: fix build error when not using nand

If CONFIG_CMD_NAND is disabled, get_nand_dev_by_index() is not
accessible.

This fix allows the build to succeed in this case.

Signed-off-by: Francois Gervais <fgervais@distech-controls.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/drivers/net/fm/fm.c b/drivers/net/fm/fm.c
index 926cf81..7a081b9 100644
--- a/drivers/net/fm/fm.c
+++ b/drivers/net/fm/fm.c
@@ -360,6 +360,7 @@
 	if (src == BOOT_SOURCE_IFC_NOR) {
 		addr = (void *)(CONFIG_SYS_FMAN_FW_ADDR +
 				CONFIG_SYS_FSL_IFC_BASE);
+#ifdef CONFIG_CMD_NAND
 	} else if (src == BOOT_SOURCE_IFC_NAND) {
 		size_t fw_length = CONFIG_SYS_QE_FMAN_FW_LENGTH;
 
@@ -372,6 +373,7 @@
 			printf("NAND read of FMAN firmware at offset 0x%x failed %d\n",
 			       CONFIG_SYS_FMAN_FW_ADDR, rc);
 		}
+#endif
 	} else if (src == BOOT_SOURCE_QSPI_NOR) {
 		struct spi_flash *ucode_flash;