drivers: ieee802154: rf2xx: Fix rf231 invalid frame

The at86rf231 frame buffer access mode read differs from all other
transceivers by only transfer one more byte after PSDU data instead
three. This difference is not evaluated in the current version of
the driver. The current change add the necessary check and read the
missing data (EQ, TRAC).

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>