[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>
diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
index 7c30531..3a7a11a 100644
--- a/drivers/of/Kconfig
+++ b/drivers/of/Kconfig
@@ -9,7 +9,7 @@
 	  OpenFirmware GPIO accessors
 
 config OF_I2C
-	def_bool y
-	depends on OF && I2C
+	def_tristate I2C
+	depends on PPC_OF && I2C
 	help
 	  OpenFirmware I2C accessors