i2c-sibyte: Mark i2c_sibyte_add_bus() as static

The i2c_sibyte_add_bus() function is not called, nor meant to, from 
outside, so mark it as static; fixing a sparse warning too.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>