libertas sdio: claim device before calling sdio_disable_func()

There is a failure path in if_sdio_power_on(), where
sdio_disable_func() is called without claiming the device.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: John W. Linville <linville@tuxdriver.com>