rtlwifi: Add check for get_btc_status callback

Drivers that do not use the get_btc_status() callback may not define a
dummy routine. The caller needs to check before making the call.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Murilo Opsfelder Araujo <mopsfelder@gmail.com>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Thadeu Cascardo <cascardo@cascardo.eti.br>
Signed-off-by: John W. Linville <linville@tuxdriver.com>