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

This patch is similar to that for bfin-uart hardware flow control.
Sport emulated serial device may be probed earlier before GPIOLIB is initialized.
Requesting and configuring CTS GPIO PIN fails in that early stage.

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>