omap-serial: Get rid of all pdev->id usage

With Device tree, pdev->id would no longer be Valid.
Hence get rid of all instances of its usage in the
driver. Device tree support for the driver is added
in subsequent patches.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Reviewed-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>