ARM: pxa: remove irq_to_gpio from ezx-pcap driver

The irq_to_gpio function was removed from the pxa platform
in linux-3.2, and this driver has been broken since.

There is actually no in-tree user of this driver that adds
this platform device, but the driver can and does get enabled
on some platforms.

Without this patch, building ezx_defconfig results in:

drivers/mfd/ezx-pcap.c: In function 'pcap_isr_work':
drivers/mfd/ezx-pcap.c:205:2: error: implicit declaration of function 'irq_to_gpio' [-Werror=implicit-function-declaration]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: stable@vger.kernel.org (v3.2+)
Cc: Samuel Ortiz <sameo@linux.intel.com>
Cc: Daniel Ribeiro <drwyrm@gmail.com>