iio: adc: at91-sama5d2_adc: fix an error code in at91_adc_allocate_trigger()

commit 73a428b3 upstream.

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.mountain
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>