rt2x00: fix TX_PWR_CFG_4 register definition

Some of the macros used to describe the TX_PWR_CFG_4 register accidentally
refer to TX_PWR_CFG_3, probably a copy&paste error. Fix that.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>