fbdev: au1200fb: Fix missing IRQ check in au1200fb_drv_probe

This func misses checking for platform_get_irq()'s call and may passes the
negative error codes to request_irq(), which takes unsigned IRQ #,
causing it to fail with -EINVAL, overriding an original error code.

Fix this by stop calling request_irq() with invalid IRQ #s.

Fixes: 1630d85a

 ("au1200fb: fix hardcoded IRQ")
Signed-off-by: Zhang Shurong <zhang_shurong@foxmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>