bridge: mdb: Split source parsing to a separate function

bridge: mdb: Split source parsing to a separate function

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 splitting the parsing of
individual attributes inside the nest to separate functions.

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