mps(4): add missing cam(4) dependency

On a MINIMAL kernel, mps.ko wouldn't load because it uses the xpt_hold_boot
symbol from CAM, but didn't have a dependency on cam(4).

(CEM: Some context: when linking loaded modules, the kernel dynamic linker
only looks for definitions in explictly marked dependency modules. Also,
the identical mpr(4) driver uses the same CAM function, but already had the
correct MODULE_DEPEND(), so no similar change is needed there.)

Submitted by: Greg V <greg AT unrelenting.technology>
Reviewed by: imp, myself
Differential Revision: https://reviews.freebsd.org/D23272