rt2x00: Remove check for rf word 0

The only way rf_write() can be called with word 0 is
when the user sends the wrong word index through debugfs.
However the values which are send through debugfs are
validated using the RF_BASE and RF_SIZE macro values,
the most logical solution is to increase RF_BASE with 4
and decrease RF_SIZE with 4 (RF_SIZE has always been
1 word too big)

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>