omci-lib-go: attributes of unknown MEs are truncated in case of extended MIB upload responses

If requested by the test, the unknown ME described below will be added to a BBSIM generated MIB Upload Next Response in extended message format:

MIB Upload Next Reponse with unknown ME added: ffff2e0b0002000000c2001a01158005fff000006400640000000080080005010801000100000000000000000000011580060000001a01158006fff000006400640000000080080006010801000100000000000000000000011580070000001a01158007fff000006400640000000080080007010801000100000000000000000000011580080000001a01158008fff00000640064000000008008000801080100010000000000000000001a0025000180000102030405060708090a0b0c0d0e0f101112131415161718191a"

001a 0025 0001 8000 0102030405060708090a0b0c0d0e0f101112131415161718191a

001a - Size of ME instance attribute values: 26
0025 - Entity class of ME instance: 37 (Intentionally left blank)
0001 - Entity instance
8000 - Attribute mask
0102030405060708090a0b0c0d0e0f101112131415161718191a - Attributes

But after decoding at ONU adapter only the first byte of the attribute values can be seen in the MIB template:

"UnknownItuG988ManagedEntity": {
"37": {
"1":

}

Corresponding adapter code can be found starting here:
https://github.com/opencord/voltha-openonu-adapter-go/blob/0edf5b0609aa8197ff131d8df4d7e816ce86d8dd/internal/pkg/mib/mib_sync.go#L678