ARM: OMAP: I2C: Fix omap_register_i2c_bus() return value on success

Commit 4d17aeb1 ("OMAP: I2C: split
device registration and convert OMAP2+ to omap_device") makes
omap2_i2c_add_bus() return a pointer to an omap_device instead on
success instead of 0.

This breaks the omap_register_i2c_bus() ABI and results in the igep0020
board code detecting an I2C bus registration error when there is none.

Fix the problem by using PTR_RET() instead of PTR_ERR() in
omap2_i2c_add_bus().

Reported-by: Alexander Kinzer <a.kinzer@plusoptix.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[tony@atomide.com: updated to return pdev instead of od]
Signed-off-by: Tony Lindgren <tony@atomide.com>