spi: orion: fix incorrect handling of cell-index DT property

In commit f814f9ac ("spi/orion: add device tree binding"), Device
Tree support was added to the spi-orion driver. However, this commit
reads the "cell-index" property, without taking into account the fact
that DT properties are big-endian encoded.

Since most of the platforms using spi-orion with DT have apparently
not used anything but cell-index = <0>, the problem was not
visible. But as soon as one starts using cell-index = <1>, the problem
becomes clearly visible, as the master->bus_num gets a wrong value
(actually it gets the value 0, which conflicts with the first bus that
has cell-index = <0>).

This commit fixes that by using of_property_read_u32() to read the
property value, which does the appropriate endianness conversion when
needed.

Fixes: f814f9ac ("spi/orion: add device tree binding")
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: <stable@vger.kernel.org> # v3.6+