serial: bfin_uart: RTS and CTS MMRs can be either 16-bit width or 32-bit width.

Change hardware flow control code to adapt to both bf5xx and bf60x.
Disabled serial device before set termios for bf60x.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>