spi-jz4740-gpio: clarify that we only support SPI mode 0, CS active-low
spi-jz4740-gpio.c: clean up a little

- remove unused PxFUNC
- make type of cast in get_nsel match literally type of function
- reorder local variables of spi_jz4740_gpio_transfer for better
  readability
spi-jz4740-gpio.c: use devres (devm_*) to simplify resource allocation

This also fixes the returning of a non-error value in some error cases.
spi-jz4740-gpio.c: don't devm_request_mem_region the port registers

The XBurst hardware makes it very easy to share port registers, so we
don't need exclusive access. We still have to worry about pin sharing,
but that's handled by gpiolib.
Merge branch 'master' of github.com:wpwrak/ben-wpan-linux

Conflicts:
	drivers/spi/spi-jz4740-gpio.c