brcmfmac: fix use of dev_kfree_skb() in irq context

The USB part of the brcmfmac did a dev_kfree_skb() that resulted
in a warning in net/core/skbuff.c:

Jul 11 04:53:33 lb-bun-10 kernel: [53282.667745] WARNING: at
net/core/skbuff.c:490 skb_release_head_state+0xcc/0xe0()

The brcmutil modules provides brcmu_pkt_buf_free_skb() which takes
the context into account. This patch makes use of this function
instead of dev_kfree_skb().

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>