usb: musb_otg_allwinner: de-constify bus_space_tags

The SAN interceptors simply take a bus_space_tag_t, so we're
dropping qualifiers here. const semantics with a ptr typedef mean we'd
have to drop or change the bus_space_tag_t abstraction used in the SAN
sanitizers in order to make a compatible change there, which likely
isn't worth it.

Reviewed by: andrew, markj
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D36764