media: winbond-cir: buffer overrun during transmit

We're reading beyond the buffer before checking its length.

BUG: KASAN: slab-out-of-bounds in wbcir_irq_tx

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>