MIPS: AR7: Use correct UART port type

PORT_AR7 has the correct TRIG flag (UART_FCR_R_TRIG_00) as well as
UART_CAP_AFE being set. This fixes UART on TNETD7300 revision 0x02,
which would otherwise mangle some characters, no side effects on
other revisions.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/1246/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>