media: ir_toy: fix a memleak in irtoy_tx

When irtoy_command fails, buf should be freed since it is allocated by
irtoy_tx, or there is a memleak.

Fixes: 4114978d

 ("media: ir_toy: prevent device from hanging during transmit")
Signed-off-by: Zhipeng Lu <alexious@zju.edu.cn>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>