i2c: omap: switch to dev_get_drvdata()

there's no need to fetch the platform_device
in order to dereference it back to the dev
pointer to access drvdata, we can use
dev_get_drvdata() instead.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Acked-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>