brcmfmac: Perform bound checking on vendor command buffer

A short or malformed vendor command buffer could cause reads outside
the command buffer.

Cc: stable@vger.kernel.org # v3.19
Signed-off-by: Pontus Fuchs <pontusf@broadcom.com>
[arend@broadcom.com: slightly modified debug trace output]
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>