drivers: i2c: Fix i2c failure on multiple slaves

Fix i2c failure while multiple slaves are connected to the
same i2c interface. Higher load on the i2c bus makes i2c
signal less nice. The requirement for signal ramp up and
ramp down time needs to be loosened.

Jira: ZEP-646

Change-Id: If698d7ba741788857694f166ecc602c2a94cb12e
Signed-off-by: Baohong Liu <baohong.liu@intel.com>