serial: bfin: ctsrts: enfore Kconfig naming convention

The CONFIG_ prefix is reserved for Kconfig options in Make and CPP
syntax; static analysis tools rely on this convention.  This patch
enforces this behavior for SERIAL_BFIN_{HARD_}CTSRTS.

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Acked-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>