e100: cleanup unneeded math

No need to convert to bytes and back - cleanup unneeded code.

Adapted from fix from 'Roel Kluin <12o3l@tiscali.nl>'

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>