ipack/devices/ipoctal: protect the channel data processing with a spinlock

We protect important data such as TX buffer pointer, nb_bytes counter and status
registers of the device, from accessing several times at the same time.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>