serial: 8250_exar: Avoid NULL pointer dereference at ->exit()

It's possible that during ->exit() the private_data is NULL,
for instance when there was no GPIO device instantiated.
Due to this we may not dereference it. Add a respective check.

Note, for now ->exit() only makes sense when GPIO device
was instantiated, that's why we may use the check for entire
function.

Fixes: 81171e7d

 ("serial: 8250_exar: Constify the software nodes")
Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20210608144239.12697-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>