zd1211rw: Fixed incorrect constant name.

Trial and error reveals that CR_ZD1211B_TX_PWR_CTL* do not affect the
transmission power.  Instead these registers seem to control the contention
windows limits for different QoS access categories.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>