spi: Use of_property_read_u32

Instead of getting the raw property, checking the length, and doing
endian conversion each time, use the OF function
of_property_read_u32() that does all that.

Error messages are slightly improved with error codes from
of_property_read_u32() for different ways the property may be invalid
(missing, too short, etc.)

Signed-off-by: Trent Piepho <tpiepho@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>