i2c: ast_i2c: Remove SCL direct drive mode

SCL direct drive mode prevents communication with devices that
do clock stretching, so disable. The Linux driver doesn't use
this mode, and the engine can handle clock stretching.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: ryan_chen <ryan_chen@aspeedtech.com>