net/ieee802154: We don't get the MFR from device drivers

commit id 7a114390 changed this.
Hopefully that changed did not affect anything.

Reducing minimal frame length and applying the change everywhere where
relevant.

Change-Id: I5ae203751bfcf70cef833620106d2c2d0e33b7a5
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>