ARM: imx: cleanup otg_mode

Cleanup the code for the otg_mode command line param:
* Use the bool type as it applies here.
* Qualify otg_mode_host with __initdata since this variable is only used in this
  context.
* The __setup functions are not supposed to return a status code, but a boolean
  indicating whether the param has been handled. See obsolete_checksetup() in
  init/main.c.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>