mwifiex: avoid deleting all stations during mwifiex_del_sta_entry()

During deleting a station entry from associated sta_list, we are
supposed to delete entry only for this particular mac address.
This patch is a bug fix wherein we were deleting all entries from
list; fix this by removing list_for_each_entry_safe() call.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>