e1000e: e1000e_cyclecounter_read(): incvalue is 32 bits, not 64

"incvalue" variable holds a result of "er32(TIMINCA) &
E1000_TIMINCA_INCVALUE_MASK" and used in "do_div(temp, incvalue)"
as a divisor.

Thus, "u64 incvalue" declaration is probably a mistake.
Even though it seems to be a harmless one, let's fix it.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>