net: sierra: shut up sparse restricted type warnings

Removes the warnings

 drivers/net/usb/sierra_net.c:343:45: warning: incorrect type in assignment (different base types)
 drivers/net/usb/sierra_net.c:343:45:    expected unsigned short [unsigned] [short] [usertype] <noident>
 drivers/net/usb/sierra_net.c:343:45:    got restricted __be16 [usertype] <noident>

and

 drivers/net/usb/sierra_net.c:658:18: warning: cast to restricted __le16

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>