i2c: Fixed i2c_dw spamming when logs are enabled

Added CONFIG_SYS_LOG_I2C_LEVEL define, so that DW I2C driver will
not print traces unless requested with CONFIG_SYS_LOG_I2C_LEVEL.
Currently it doesn't obey it, so DW I2C spams DBG traces when
logging is enabled.

Change-Id: I87dc1d922ba61d39f26945627602864c75d9bf33
Signed-off-by: Jani Pajarinen <jani.pajarinen@intel.com>