rt2x00: Fix calculation of rt2800 iveiv entry offset.

Fix typo. The index should be multiplied by the entry size, not 'and'-ed.

Found via code-inspection.

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