spi: pxa2xx: Use ACPI_COMPANION() instead of acpi_bus_get_device()

Get pointer to the struct acpi_device by using ACPI_COMPANION() macro. This
is more efficient than using ACPI_HANDLE() and acpi_bus_get_device().

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>