mwifiex: avoid memsetting PCIe event buffer

Preallocated PCIe buffer is being reused for all PCIe interface
events. Physical address of the buffer is shared with firmware
so that it can perform DMA on it. As event length is specified
in the header, there should not be a problem if the buffer gets
overwritten.
We will save some cycles by avoiding memset everytime while
submitting the buffer to firmware.

Fixes: 2728cecd

(mwifiex: corrections in PCIe event skb)
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>