pallet-beefy-mmr: fix when using custom hashing (#14273)

`Hash` is no longer part of `pallet_mmr::Config`, but this code
would still build because it assumes we're referring to underlying
`<pallet_mmr::Config as frame_system::Config>::Hash` which is not
what we want when using different hashing between `frame_system`
and `pallet_mmr`.

Disambiguate by using correct `pallet_mmr::Config::Hashing` type.

Signed-off-by: acatangiu <adrian@parity.io>