ice: Resolve static analysis reported issue

Static analysis points out the default case in the switch statement in
ice_get_itr_intrl_gran() is an infeasible condition causing the default
case statement to be unreachable.  Remove it and since the function no
longer returns anything but success, change it to just return void and
update the only call to it accordingly.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>