Do no reprobe for a supported protocol when enabling a pointing

Do no reprobe for a supported protocol when enabling a pointing
device of type PMS_STANDARD, probing during autoconf(9) is enough!.

In such case, the device can be a simple PS/2 mouse, a unsupported
touchpad or a downgraded touchpad if something bad happened.  But
it is very unlikely that reprobing will improve the situation and
transform a standard mouse into a multitouch-aware touchpad ;)

This fixes the 12 seconds delay seen on various Dell laptops (E4310
and Latitude D630) when starting Xorg, reported by Kārlis Miķelsons
on bugs@.

ok stsp@, dcoppa@, shadchin@