net: mvneta: Drop unneeded macro

Macro MVNETA_GMAC_FORCE_LINK_UP can be dropped from value assignment in
fixed link case, since it's value is written into the register later in
the function for link-down-to-link-up case. The value is written as
MVNETA_GMAC_FORCE_LINK_DOWN | MVNETA_GMAC_FORCE_LINK_PASS, and so the
macro definition can also be dropped.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>