mwifiex: fix 'Smatch' warnings

Following three warnings are fixed:

>init.c +256 mwifiex_init_adapter(71)
>warn: variable dereferenced before check 'adapter->sleep_cfm'
>(see line 191)

>sta_rx.c +193 mwifiex_process_sta_rx_packet(75)
>warn: variable dereferenced before check 'priv' (see line 182)

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>