staging: vchiq: Fix bulk userdata handling

The addition of the local 'userdata' pointer to
vchiq_irq_queue_bulk_tx_rx omitted the case where neither BLOCKING nor
WAITING modes are used, in which case the value provided by the
caller is replaced with a NULL.

Fixes: 4184da4f ("staging: vchiq: fix __user annotations")

Signed-off-by: Phil Elwell <phil@raspberrypi.com>