nvidiafb: Correctly assign the i2c class with the port reversal

If the i2c ports are to be reversed, I2C_CLASS_HWMON assignment must also be
reversed.

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>