port: report link role and negotiated pause per port

Two per-port MAC status registers that nothing in the firmware reads:
0x6408 says which end is master on a BASE-T link, 0x63FC and 0x6400 say
whether pause was negotiated in each direction. port <n> show prints
both at the end.

The read sits in rtl837x_port.c rather than phy_show(), because these
are MAC registers and phy_show() is skipped for SFP ports. Master/slave
keys off machine.is_sfp[] so it stays off the fibre ports, where the
notion does not apply.

Bit N of each register is chip port N, which is not how
RTL837X_REG_LINKS_STS indexes its bytes a few lines above in the same
header.

rtl837x_port.h

@@ -49,6 +49,7 @@ extern __xdata struct vlan_settings vlan_settings;
 uint8_t port_l2_forget(void) __banked;
 void port_l2_learned(void) __banked;
 void port_stats_print(void) __banked;
+void port_media_show(uint8_t port) __banked;
 int8_t vlan_get(uint16_t vlan) __banked;
 __xdata uint16_t vlan_name(uint16_t vlan) __banked;
 void vlan_name_remove(uint16_t vlan) __banked;