mlxsw: spectrum_router: Create common function for fib_entry_type_unset() code

mlxsw_sp_fib4_entry_type_unset() is not specific for IPv4 FIB entry,
move the code to mlxsw_sp_fib_entry_type_unset(), and call this function
from mlxsw_sp_fib4_entry_type_unset() so then it will be used for IPv6
also.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>