i2c: designware: Fix standard mode speed when configuring the slave mode

Code sets bit DW_IC_CON_SPEED_FAST (0x4) always when configuring the slave
mode. This results incorrect register value DW_IC_CON_SPEED_HIGH (0x6)
when OR'ed together with DW_IC_CON_SPEED_STD (0x2).

Remove this and let the code set the speed mode bits according to clock
frequency or default to fast mode.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>