ipack/devices/ipoctal: add rx_enable flag

Thus, we don't enable RX when a termios setup has been called, as it could be
disabled previously.

As the control registers (Rx, Tx flags specifically) cannot be read from the
device, we keep this info in rx_enable.

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