serial: bfin_sport_uart: only enable SPORT TX if data is to be sent

Rather than always turn on the SPORT TX interrupt, only do it when we've
actually queued up data for transmission.  This avoids useless interrupt
processing.

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>