drivers: ethernet: xlnx_gem: fix dev data access missed by #41918

Fixes a compiler error in eth_xlnx_gem_stats() caused by the
automated replacement of device run-time data pointer accesses
by #41918. The device run-time data access in eth_xlnx_gem_stats()
was likely missed as this function is not usually compiled - its
existance depends on CONFIG_NET_STATISTICS_ETHERNET being set.

Signed-off-by: Immo Birnbaum <Immo.Birnbaum@weidmueller.com>