brcm80211: smac: eliminate a null pointer dereference in dma.c

Though it's unlikely, di may be null, so we can't dereference
di->dma.dmactrlflags until we've checked it.

Move this de-reference after the check, and adjust the error
message to not require de-referencing di.

This is based upon Julia's original patch:
<1319846297-2985-2-git-send-email-julia@diku.dk>

Reported-by: Julia Lawall <julia@diku.dk>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>