serial: bfin_5xx: fix typo in IER check

This most likely won't cause problems on systems as people don't typically
enable GPIO RTS/CTS if they don't actually use it.

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>