soc/intel/broadwell/lpc.c: don't zeroize existing gnvs table

soc/intel/broadwell/lpc.c: don't zeroize existing gnvs table

The gnvs table only needs to be zeroized after init;
zeroizing an existing/populated table renders all I2C devices
completely non-functional.

TEST: boot Linux and observe all I2C devices functional

Change-Id: Id149ad645dfe5ed999a65d10e786e17585abc477
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/17828
Tested-by: build bot (Jenkins)
Reviewed-by: Nico Huber <nico.h@gmx.de>