am53c974: Fix crash during modprobe

On systems with shared interrupts the interrupt routine might
be called as soon as the interrupt is enabled.
As this might happen before pci_set_drvdata() is called the
system would crash.

Reported-by: Andreas Brogle <anbro@ok.de>
Tested-by: Andreas Brogle <anbro@ok.de>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>