iio: adc: mcp3911: return proper error code on failure to allocate trigger

iio: adc: mcp3911: return proper error code on failure to allocate trigger

smatch warnings:
drivers/iio/adc/mcp3911.c:441 mcp3911_probe() warn: passing zero to 'PTR_ERR'

Fixes: 08a65f61db69 ("iio: adc: mcp3911: add support for interrupts")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Link: https://lore.kernel.org/r/20220927092537.94663-1-marcus.folkesson@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>