e1000: Use kcalloc()

Replace kmalloc+memsetout the driver. Slightly modified by Auke Kok.

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