spi: fsl-espi: improve return value handling in fsl_espi_probe

The return value of fsl_espi_probe (currently struct spi_master *)
is just used for checking whether an error occurred.
Change the return value type to int and simplify the code.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>