sets GEM_MAC_RX_CONFIG based on sc_mac_rxcfg which in turn is initialized
to zero, before reading the supposedly default configuration we were
effectively not basing sc_mac_rxcfg and thus GEM_MAC_RX_CONFIG on the
default configuration. Solve this by calling gem_setladrf() after reading
in the default configuration of GEM_MAC_RX_CONFIG. This also avoids the
need to distinguish whether gem_setladrf() should enable the RX MAC again
and should be slightly more correct as we're now doing all of the RX MAC
configuration in the intended step.