Input: ad7879 - add check for read errors in interrupt

regmap_bulk_read() can return a non zero value on failure. The fix checks
if the function call succeeded before calling mod_timer. The issue was
identified by a static analysis tool.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>