mwifiex: fix null derefs, mem leaks and trivia

This patch:
 - adds kfree() where necessary
 - prevents potential null dereferences
 - makes use of kfree_skb()
 - replaces -1 for failed kzallocs with -ENOMEM

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Reviewed-by: Kiran Divekar <dkiran@marvell.com>
Tested-by: Amitkumar Karwar <akarwar@marvell.com>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>