Properly set the low watermarks when reducing the baud rate.

Properly set the low watermarks when reducing the baud rate.

Now that buffers are deallocated lazily, we should not use
tty*q_getsize() to obtain the buffer size to calculate the low
watermarks. Doing this may cause the watermark to be placed outside the
typical buffer size.

This caused some regressions after my previous commit to the TTY code,
which allows pseudo-devices to resize the buffers as well.

Reported by:	yongari, dougb
MFC after:	1 week