serial: bfin_5xx: need to disable DMA TX interrupt too

If we don't disable the DMA TX channel, an inopportune timeout will
trigger the interrupt handler and may cause a dead lock with the spin_lock.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>