serial: imx: fix NULL dereference Oops when pdata == NULL

The platform code doesn't have to provide platform data to get sensible
default behaviour from the imx serial driver.

This patch does not handle NULL dereference in the IrDA case, which still
requires a valid platform data pointer (in imx_startup()/imx_shutdown()),
since I don't know whether there is a sensible default behaviour, or
should the operation just fail cleanly.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Oskar Schirmer <os@emlix.com>
Cc: Fabian Godehardt <fg@emlix.com>
Cc: Daniel Glöckner <dg@emlix.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>