net: marvell: prestera: fix NULL dereference on devlink_alloc() failure

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

net: marvell: prestera: fix NULL dereference on devlink_alloc() failure

devlink_alloc() may return NULL on allocation failure, but prestera_devlink_alloc() unconditionally calls devlink_priv() on the returned pointer.

This leads to a NULL pointer dereference if devlink allocation fails. Add a check for a NULL devlink pointer and return NULL early to avoid the crash.