mtd: cfi_flash: fix clang warning

clang warns this check is silly; it is since s is
a local variable.

u-boot/drivers/mtd/cfi_flash.c:2363:13: warning: comparison of
  array 's' not equal to a null pointer is always true
  else if ((s != NULL) && (strcmp(s, "yes") == 0)) {

cc: Stefan Roese <sr@denx.de>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>