staging:iio:adc:ad7152 remove unregister of interrupt line.

a) This interface is going away
b) There is no matching register call so looks like a cut and paste
error

Trivial sparse warning fix.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>