mtd: physmap_of: fix set but unused warning

drivers/mtd/maps/physmap_of.c: In function ‘of_flash_probe’:
drivers/mtd/maps/physmap_of.c:165:16: warning: variable ‘p’ set but not used [-Wunused-but-set-variable]

This could be a problem if the 'reg' property is not set, since that
means 'count' will be uninitialized.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>