mfd: cros_ec: Free IRQ on exit

Currently we request the irq when probing, but never free it. So after
unbind ec driver, this irq will be left requested, which would break
the next bind:

[ 2683.338437] genirq: Flags mismatch irq 64. 00002008 (chromeos-ec) vs. 00002008 (chromeos-ec)
[ 2683.338591] cros-ec-spi spi5.0: request irq 64: error -16
[ 2683.338610] cros-ec-spi spi5.0: cannot register EC
[ 2683.338656] cros-ec-spi: probe of spi5.0 failed with error -16

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>