Staging: ipack/devices/ipoctal: read more than one character from RX FIFO.

The RX FIFO has a size of 3 characters. Check if when we are picking the
oldest one, we have more to read.

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