Fix a typo in the PMAP_PTE_SET_CACHE_BITS macro.

The second argument should have been "pa" not "ps". It worked by
accident because the argument was always "pa" which was an in-scope
local variable.

Submitted by: sson
Reviewed by: jhb, kevans
Obtained from: CheriBSD
MFC after: 3 days
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D22338