serial: bfin-uart: Remove ASYNC_CTS_FLOW flag for hardware automatic CTS.

Blackfin uart supports automatic CTS trigger when hardware flow control is enabled.
No need to start and top tx in CTS interrupt. So, remote ASYNC_CTS_FLOW flag.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>