mwifiex: remove redundant local variables and comments

Remove some local variables (mainly function return values)
that are used only once. Also, one dummy function and some
wordy comments are removed.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>