rt2x00: Remove suspicious register write

Remove suspicious register write as the reg variable is never filled
with an TX_SW_CFG2 associated value before.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>