mwifiex: NULL check for cfg80211_inform_bss()

cfg80211_inform_bss would return null in some cases, such as
memory allocation failure.

This patch adds sanity check for this case, to avoid possible
issues when above corner case is hit.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>