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

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[email protected]>

Reported-by: Julia Lawall <[email protected]>
Acked-by: Arend van Spriel <[email protected]>
Signed-off-by: Julian Calaby <[email protected]>
Signed-off-by: John W. Linville <[email protected]>