irda: add missing BKL in irnet_ppp ioctl

One ioctl has been forgotten when the BKL was push down into irnet_ppp
ioctl function.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>