serial: 8250_lpss: avoid potential kernel crash when remove

This is a follow up to the commit a9b01b58 ("serial: 8250_mid fix calltrace
when hotplug 8250 serial controller") in which the kernel crash was described
for another 8250 based driver. It appears that we have the very same issue in
8250_lpss. Fix it by unregistering serial driver first.

Cc: Liwei Song <liwei.song@windriver.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>