Use unsigned loads in fubyte, fuword16, generic_bs_r_1, generic_bs_r_2
as these functions should do zero-extend.

Discovered by running pci_read_cap(), and by hint from James Clarke.

Reviewed by: James Clarke <jrtc27@jrtc27.com>
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D23236