rt2800: correct BBP1_TX_POWER_CTRL mask

Two bits control TX power on BBP_R1 register. Correct the mask,
otherwise we clear additional bit on BBP_R1 register, what can have
unknown, possible negative effect.

Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>