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

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.

Fixes: 34dd1710f5a3 ("net: marvell: prestera: Add basic devlink support")
Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Acked-by: Elad Nachman <enachman@marvell.com>
Link: https://patch.msgid.link/20251230052124.897012-1-alok.a.tiwari@oracle.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>