staging: csr: fix the reset of skb mac header

mac_header can be offset if NET_SKBUFF_DATA_USES_OFFSET is set, so
we should call skb_reset_mac_header to reset mac_header.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>