mt76: add LINUXKPI_PARAM_PREFIX for module_param*
In order to not overwrite sysctl/tunables under compat.linuxkpi
we need to prefix the module_param* names with a per-driver/file
designator to make them (more) uniq. Add the FreeBSD specific
LINUXKPI_PARAM_PREFIX defines for that where missing in mt76.
Sponsored by: The FreeBSD Foundation
(cherry picked from commit 853e0440c97a4a1797edd6d653fba4398dc885e8)