mtd: denali: Mark function is_erased() as static

This patch marks the function is_erased() as static in denali.c because
it is not used outside this file.

This patch elimiates the following warning in nand/denali.c:
drivers/mtd/nand/denali.c:900:6: warning: no previous prototype for ‘is_erased’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/nand/denali.c

@@ -897,7 +897,7 @@ static void read_oob_data(struct mtd_info *mtd, uint8_t *buf, int page)
 /* this function examines buffers to see if they contain data that
  * indicate that the buffer is part of an erased region of flash.
  */
-bool is_erased(uint8_t *buf, int len)
+static bool is_erased(uint8_t *buf, int len)
 {
 	int i = 0;
 	for (i = 0; i < len; i++)