mwifiex: fix cmd_skb headroom decreasing issue

Before calling host_to_card() to send the cmd to firmware,
we use skb_push() to add 4 bytes SDIO interface header at
the start of the data buffer. Since cmd_skb data structure
will be re-used at a later time, we need to restore its
headroom by removing the 4 bytes header.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Marc Yang <yangyang@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>