mwifiex: pcie: tighten a check in mwifiex_pcie_process_event_ready()

If "evt_len" is 1 then we try to memcpy() negative 3 bytes and it would
cause memory corruption.

Fixes: d930faee ("mwifiex: add support for Marvell pcie8766 chipset")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>