ice: Update comment regarding the ITR_GRAN_S

Since the driver now hard codes the ITR granularity to 2 us in the
GLINT_CTL register the comment next to ITR_GRAN_S needs to be updated.

Signed-off-by: Brett Creeley <brett.creeley@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>