iio: adc: at91-sama5d2_adc: fix probe error on missing trigger property

This fix allows platforms to probe correctly even if the
trigger edge property is missing. The hardware trigger
will no longer be registered in the sybsystem
Preserves backwards compatibility with the support that
was in the driver before the hardware trigger.

https://storage.kernelci.org/mainline/master/v4.14-rc2-255-g74d83ec2b734/arm/sama5_defconfig/lab-free-electrons/boot-at91-sama5d2_xplained.txt



Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Fixes: 5e1a1da0 ("iio: adc: at91-sama5d2_adc: add hw trigger and buffer support")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>