mwifiex: fix wakeup on magic packet

 8 bytes preamble
14 bytes src/dst/eth_type
 6 bytes 0xff:0xff..

http://en.wikipedia.org/wiki/Wake-on-LAN#Magic_packet
http://en.wikipedia.org/wiki/EtherType

This will fail if we VLAN or the magic packet is encapsulated
as a UDP packet...

Signed-off-by: Andreas Fenkart <afenkart@gmail.com>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>