spi: pxa2xx: Factor out handle_bad_msg

As suggested by Andy Shevchenko: Decouple this corner cause from the
general handling logic in ssp_int.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>