mtd: bfin-async-flash: Remove unused variable

Avoid the following compile warning:

drivers/mtd/maps/bfin-async-flash.c: In function 'bfin_flash_probe':
drivers/mtd/maps/bfin-async-flash.c:129: warning: unused variable 'ret'

Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>