libertas: remove CMD_802_11_PWR_CFG

This has nowhere been used. Note: in the firmware manual this was
documented as CMD_802_11_PA_CFG. If we ever need it, we can/should
re-implement it as a direct command.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>