ionic: catch failure from devlink_alloc

In the Linux kernel, the following vulnerability has been resolved:

ionic: catch failure from devlink_alloc

Add a check for NULL on the alloc return. If devlink_alloc() fails and

we try to use devlink_priv() on the NULL return, the kernel gets very

unhappy and panics. With this fix, the driver load will still fail,

but at least it won't panic the kernel.