Blackfin SPORT UART: fix data misses while using transmit frame sync

SPORT transmit frame sync (TFS) isn't used as an electrical signal during
normal SPORT UART emulation.  However, it is useful in EIA RS-485
emulation as RS-485 Transceiver Driver Enable DE strobe.

This patch configures:
TFS to be active high in order to drive an DE strobe of
an eventually connected RS-485 Transceiver.

Late frame sync mode (LATFS) gating the entire TX shift cycle.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>