realtek: 5.15: replace fallthrough comment for rtl838x ethernet driver

realtek: 5.15: replace fallthrough comment for rtl838x ethernet driver

Replace fallthrough comment with fallthrough macro for rtl838x ethernet
driver.
Fix compilarion warning:
drivers/net/ethernet/rtl838x_eth.c: In function 'rtl930x_mdio_reset':
drivers/net/ethernet/rtl838x_eth.c:1959:43: error: this statement may fall through [-Werror=implicit-fallthrough=]
 1959 |                         private_poll_mask |= BIT(i);
drivers/net/ethernet/rtl838x_eth.c:1961:17: note: here
 1961 |                 case PHY_INTERFACE_MODE_USXGMII:
      |                 ^~~~

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>