uart(4) 16550A FIFO latency too long

[UEMURA Tetsuya]

Hi there.

I have a timing sensitive sensor device connected to PC with serial
ports. In order to gather data from the device correctly, I have to set
the 16550A FIFO interrupt trigger level lower than the default
FCR_RX_MEDH to FCR_RX_MEDL or FCR_RX_LOW.

However, there's no tunable in uart(4) to set these levels manually,
so I've added some lines to uart(4) and now I've been able to set the
level via device.hints.

Please someone review the following PR.
http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/121421 .

Thank in advance.

-- 
UEMURA Tetsuya <t_uemura at macome.co.jp>