[POWERPC] i2c: Fix build breakage introduced by OF helpers

Fix build breakage introduced in commit "[POWERPC] i2c: OF helpers for
the i2c API".  If i2c-core is compiled as a module, the helper needs
to be compiled as a module, as well.  Rename i2c.c to of_i2c.c to
avoid name space conflict.

[paulus@samba.org: Changed dependency from OF to PPC_OF to avoid
sparc{32,64} allmodconfig breakage.]

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>