fbdev: omapfb: Do not shadow error code from platform_get_irq()

There is no point in shadowing the error codes from platform_get_irq().
Refactor omapfb_do_probe() accordingly.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Helge Deller <deller@gmx.de>