USB: serial: f81534: fix hang-up on overrun

The F81532/534 without this patch will hang-up on data overrun.

It's caused by enable LSR interrupt in IER by default and occur data
overrun, the chip will busy for process LSR interrupt but not read LSR
internally. It will not responed for USB control endpoint0 and we can't
read LSR from driver in this situration.

So we'll disable the LSR interrupt in probe() and submit the LSR worker to
clear LSR state when reported LSR error bit with bulk-in data in
f81534_process_per_serial_block().

Signed-off-by: Ji-Ze Hong (Peter Hong) <hpeter+linux_kernel@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>