net: ethernet: 3com: 3c574_cs: Remove set but unused variables 'tx' and 'rx'

Fixes the following W=1 kernel build warning(s):

 drivers/net/ethernet/3com/3c574_cs.c: In function ‘update_stats’:
 drivers/net/ethernet/3com/3c574_cs.c:954:9: warning: variable ‘tx’ set but not used [-Wunused-but-set-variable]
 954 | u8 rx, tx, up;
 | ^~
 drivers/net/ethernet/3com/3c574_cs.c:954:5: warning: variable ‘rx’ set but not used [-Wunused-but-set-variable]
 954 | u8 rx, tx, up;
 | ^~

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Shannon Nelson <snelson@pensando.io>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Cc: Martin Habets <mhabets@solarflare.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Donald Becker <becker@scyld.com>
Cc: David Hinds <dahinds@users.sourceforge.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>