fastboot: Use const qualifier for char *part_name

In fastboot_*_get_part_info() functions we can use stronger typing by
expecting const strings.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Igor Opaniuk <igor.opaniuk@toradex.com>
diff --git a/drivers/fastboot/fb_mmc.c b/drivers/fastboot/fb_mmc.c
index 90ca81d..0a335db 100644
--- a/drivers/fastboot/fb_mmc.c
+++ b/drivers/fastboot/fb_mmc.c
@@ -298,7 +298,8 @@
  * @part_info: Pointer to returned disk_partition_t
  * @response: Pointer to fastboot response buffer
  */
-int fastboot_mmc_get_part_info(char *part_name, struct blk_desc **dev_desc,
+int fastboot_mmc_get_part_info(const char *part_name,
+			       struct blk_desc **dev_desc,
 			       disk_partition_t *part_info, char *response)
 {
 	int r;