media: ir_toy: deal with residual irdata before expected response

After sending the start transmit command, the device is supposed to
respond with the length of the buffer which can be sent. There might
be some residual ir data there.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>