spi/pxa2xx: fix compile warning in pxa2xx_spi_acpi_get_pdata()

Commit cbfd6a21 (spi/pxa2xx: Convert to devm_ioremap_resource())
converted the driver to use devm_ioremap_resource(). However it causes
following warning to be emitted:

drivers/spi/spi-pxa2xx.c: In function ‘pxa2xx_spi_acpi_get_pdata’:
drivers/spi/spi-pxa2xx.c:1094:3: warning: return makes pointer from integer without a cast [enabled by default]

Fix this by returning NULL as it was done previously (error printing is
already done by devm_ioremap_resource()).

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>