i2c: use macro IS_ENABLED in header i2c.h

Using the macro IS_ENABLED to check the option CONFIG_I2C=(y|m) makes
the code nicer.  No functional change.

Signed-off-by: Stefan Lengfeld <contact@stefanchrist.eu>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
1 file changed