cam: Check if cam_simq_alloc fails for the xpt bus during module init

This is very unlikely to fail (and if it does, CAM isn't going to work
regardless), but fail with an error rather than a gauranteed panic via
NULL pointer dereference.

PR: 276770
Reported by: Qiushi <w290680224@gmail.com>

(cherry picked from commit eb86c6c5b462c996e44c45ba496937b75ef22da3)