mwifiex: fix null pointer deference when adapter is null

If adapter is null the error exit path in mwifiex_shutdown_sw is
to down the semaphore sem and print some debug via mwifiex_dbg.
However, passing a NULL adapter to mwifiex_dbg causes a null
pointer deference when accessing adapter->dev.  This fix checks
for a null adapter at the start of the function and to exit
without the need to up the semaphore and we also skip the debug
to avoid the null pointer dereference.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>