net: ipv6: scope checking function fix

Change the implementation of net_ipv6_is_addr_mcast_scope() inline
function that let us check if a given IPv6 address has a specified
scope. Previously, it was comparing the whole byte including flags
of a multicast address. It meant, that while checking for a specific
scope a one was also checking the flags. Even in Zephyr's net stack
there are checks for a IPv6 link local scope that are failing for
addresses that are not marked as "well known" (when least significant
bit of the flags is set).

Signed-off-by: Konrad Derda <konrad.derda@nordicsemi.no>