kbd drivers: don't double register keyboard drivers

Keyboard drivers are generally registered via linker set. In these cases,
they're also available as kmods which use KPI for registering/unregistering
keyboard drivers outside of the linker set.

For built-in modules, we still fire off MOD_LOAD and maybe even MOD_UNLOAD
if an error occurs, leading to registration via linker set and at MOD_LOAD
time.

This is a minor optimization at best, but it keeps the internal kbd driver
tidy as a future change will merge the linker set driver list into its
internal keyboard_drivers list via SYSINIT and simplify driver lookup by
removing the need to consult the linker set.