brcmfmac: Fix use after free in brcmf_sdio_readframes()

The brcmu_pkt_buf_free_skb() function frees "pkt" so it leads to a
static checker warning:

    drivers/net/wireless/broadcom/brcm80211/brcmfmac/sdio.c:1974 brcmf_sdio_readframes()
    error: dereferencing freed memory 'pkt'

It looks like there was supposed to be a continue after we free "pkt".

Fixes: 4754fcee

 ("brcmfmac: streamline SDIO read frame routine")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>