smsc95xx: fix smsc_crc return type

This patch fixes a bug introduced in bbd9f9ee

 which could prevent
some wakeups from working correctly if multiple wol options were
selected.

This helper function calculates a 16-bit crc and shifts it into
either the high or low 16 bits of a u32 so the caller can or it
directly into place.  The function previously had a u16 return
type so would always have returned zero when filter was odd.

Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Reported-by: Bjorn Mork <bjorn@mork.no>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>