serial: bfin-uart: Request CTS GPIO PIN when the serial device starts up.

Serial device may be probed earlier before GPIOLIB is initialized. Requesting and
configuring CTS GPIO PIN fails in that early stage. Do it when the serial device
really starts up.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>