ft232: support the ASYNC_LOW_LATENCY flag

This allows users to use the standard setserial command with this FT232
feature as well as obscure chip specific interfaces we have now. We keep
track of and respect the sysfs value for non-low-latency cases. In theory we
could do smart stuff with VTIME and the like but this seems of questionable
worth.

Closes-bug: http://bugzilla.kernel.org/show_bug.cgi?id=9120
Signed-off-by: Alan Cox <alan@linux.intel.com)>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>