brcmfmac: fix leak of mypkt on error return path

Currently if the call to brcmf_sdiod_set_backplane_window fails then
error return path leaks mypkt. Fix this by returning by a new
error path labelled 'out' that calls brcmu_pkt_buf_free_skb to free
mypkt.  Also remove redundant check on err before calling
brcmf_sdiod_skbuff_write.

Addresses-Coverity: ("Resource Leak")
Fixes: a7c3aa15

 ("brcmfmac: Remove brcmf_sdiod_addrprep()")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>