The at91_adc_allocate_trigger() function is supposed to return error
pointers.  Returning a NULL will cause an Oops.

Fixes: 5e1a1da0 ("iio: adc: at91-sama5d2_adc: add hw trigger and buffer support")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/5d728f9d-31d1-410d-a0b3-df6a63a2c8ba@kili.mountainSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>