spi: pxa2xx: Convert unique ID string of ACPI device as unsigned integer

Andy noticed numeric unique device ID is unsigned integer so convert it
using kstrtouint(). Actually integer in ACPI 2.0 and later is 64 bits
litte-endian unsigned integer but quite certainly having so big value here
would mean something extra than just the SPI bus number so it won't hurt to
assume only lower 32 bits carry the bus number for now.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>