mwifiex: code rearrangement for better readability

Use negative check (if(!bss_desc)) and return failure
instead of failing a NULL check later in
mwifiex_check_network_compatibility() routine.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>