ARM: pxa: fix typo 'CONFIG_SPI_PXA2XX_MASTER'

CONFIG_SPI_PXA2XX_MASTER was used were it was surely meant to use
CONFIG_SPI_PXA2XX_MODULE. Use the IS_ENABLED() macro here, as it guards
against typos like this one.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Haojian Zhuang <haojian.zhuang@linaro.org>