spi: fsl-lpspi: Clean up fsl_lpspi_probe

Use is_slave local variable to avoid calling of_property_read_bool() twice.
Remove redudant assignment for controller->bus_num, set it once is enough.
Move setting controller->bits_per_word_mask close to the code init other
controller fields.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>