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

stable inclusion
from stable-v5.10.179
commit 42604b4ad5f3289596455c786c04c67a1b5740d9
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8C809

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=42604b4ad5f3289596455c786c04c67a1b5740d9



--------------------------------

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>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>