mwifiex: release bss structure returned by cfg80211_inform_bss()

Following compilation warning is fixed by releasing referenced BSS
structure returned by cfg80211_inform_bss().

"warning: ignoring return value of cfg80211_inform_bss,
declared with attribute warn_unused_result"

Cc: Johannes Berg <johannes.berg@intel.com>
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>