Input: ad714x - fix threshold and completion interrupt masks

Fix two issues in the threshold and completion interrupt mask and unmask
functions. According to the AD714x datasheets the highest stage completion
interrupt should be enabled. Fix mask computation.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Tested-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>