bridge: mdb: Use a boolean to indicate nest is required

bridge: mdb: Use a boolean to indicate nest is required

Currently, the only attribute inside the 'MDBA_SET_ENTRY_ATTRS' nest is
'MDBE_ATTR_SOURCE', but subsequent patches are going to add more
attributes to the nest.

Prepare for the addition of these attributes by determining the
necessity of the nest from a boolean variable that is set whenever one
of these attributes is parsed. This avoids the need to have one long
condition that checks for the presence of one of the individual
attributes.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David Ahern <dsahern@kernel.org>