ath5k: fix off-by-one in pilot magnitude mask

If the symbol offset is 46, it will be counted in both
the third and fourth bytes of the mask, and in this
case the shift will be negative which can pollute
high order bits in the mask.  This may negatively impact
OFDM symbol detection.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>