spi: spi-fsl-spi: Return an error code in fsl_spi_do_one_msg()

Since commit c592becb ("spi: fsl-(e)spi: migrate to generic master
queueing") the function fsl_spi_do_one_msg() is not void anymore, so return
an error code to avoid the following buid warning:

   drivers/spi/spi-fsl-spi.c: In function 'fsl_spi_do_one_msg':
>> drivers/spi/spi-fsl-spi.c:374:4: warning: 'return' with no value, in function returning non-void [-Wreturn-type]
       return;
       ^

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>