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>