serial: bfin_sport_uart: shorten the SPORT TX waiting loop

The waiting loop to stop SPORT TX from TX interrupt is too long.  This may
block the SPORT RX interrupts and cause the RX FIFO to overflow. So, do
stop sport TX only after the last char in TX FIFO is moved into the shift
register.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>