fm10k/base: use bit shift for ITR scale

Use bitshift instead of a divisor, because this is faster, and
eliminates any need for a '0' check. In our case, this even works
out because default Gen3 will be 0.

Because of this, we are also able to remove the check for non-zero value
in the VF code path since that will already be the default Gen3 case.

Signed-off-by: Wang Xiao W <xiao.w.wang@intel.com>
Tested-by: Heng Ding <hengx.ding@intel.com>
Acked-by: Jing Chen <jing.d.chen@intel.com>