spi: pxa2xx: avoid redundant gpio_to_desc(desc_to_gpio()) round-trip

gpio_free(gpio) simply does gpiod_free(gpio_to_desc(gpio)), so it's
simpler and cleaner to use gpiod_free directly.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Mark Brown <broonie@kernel.org>