rndis_wlan: remove CAP_SUPPORT_TXPOWER/OID_802_11_TX_POWER_LEVEL code

BCM4320 doesn't support OID_802_11_TX_POWER_LEVEL (chip implements
the command but setting value has no effect and getting txpower value
always returns 0xff, full power). So remove the code for cleanup.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>