Fix the following warning:

drivers/mtd/chips/cfi_util.c:112:3: warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]
                   fallthrough;
                   ^
   include/linux/compiler_attributes.h:210:41: note: expanded from macro 'fallthrough'
   # define fallthrough                    __attribute__((__fallthrough__))
                                           ^
   drivers/mtd/chips/cfi_util.c:168:3: warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]
                   fallthrough;
                   ^
   include/linux/compiler_attributes.h:210:41: note: expanded from macro 'fallthrough'
   # define fallthrough                    __attribute__((__fallthrough__))

by placing the fallthrough; statement inside ifdeffery.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>