tty: hvc_haven: Use thread worker to send characters

hh_hvc_put_chars may be called with interrupts disabled.
hh_rm_console_write is a blocking call (to wait for the response to come
back). Use a FIFO and workqueue to send characters so that console_write
may sleep.

Change-Id: I7060b3604cb112caf00ecc1d94a5b2d543e7956e
Signed-off-by: Elliot Berman <eberman@codeaurora.org>