brcmsmac: Free packet if dma_mapping_error() fails in dma_rxfill

In case dma_mapping_error() returns an error in dma_rxfill, we would be
leaking a packet that we allocated with brcmu_pkt_buf_get_skb().

Reported-by: coverity (CID 1081819)
Fixes: 67d0cf50

 ("brcmsmac: Fix WARNING caused by lack of calls to dma_mapping_error()")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>