sym(4): Provide a DEVICE_DETACH(9) method

This also fixes sym_cam_free() to tear things down correctly, i. e. in
opposite order of setup, as well as sym_cam_attach() to not free devq
and SIM twice in the failure case.

(cherry picked from commit 518c01b31e96bbe8353a551885a61e7da6aed5f6)