blackfin: fix wrong CTS inversion

The Blackfin serial headers were inverting the CTS value leading to wrong
handling of the CTS line which broke CTS/RTS handling completely.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>