mtd: cfi: remove unneeded NULL checks

In cfi_intelext_setup and cfi_amdstd_setup, mtd is never NULL.
Remove unnecessary checks.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>