mwifiex: corrections in PCIe event skb handling

Preallocated event SKBs are getting reused for PCIe chipset.
Their physical addresses are shared with firmware so that
firmware can write data into them.

This patch makes sure that SKB is cleared and length is set to
default while submitting it to firmware.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Zhaoyang Liu <liuzy@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>