USB: serial: f81232: fix interrupt worker not stop

The F81232 will use interrupt worker to handle MSR change.
This patch will fix the issue that interrupt work should stop
in close() and suspend().

This also fixes line-status events being disabled after a suspend cycle
until the port is re-opened.

Signed-off-by: Ji-Ze Hong (Peter Hong) <hpeter+linux_kernel@gmail.com>
[ johan: amend commit message ]
Fixes: 87fe5adc ("USB: f81232: implement read IIR/MSR with endpoint")
Cc: stable <stable@vger.kernel.org>	# 4.1
Signed-off-by: Johan Hovold <johan@kernel.org>