Bluetooth: host: disallow MPS decrease for multiple CIDs in Reconf Req

When in le_ecred_reconf_req multiple CIDs are received host should
return  0x0002 error (Reconfiguration failed - reduction in size of
MPS not allowed for more than one channel at a time).

This affects test L2CAP/ECFC/BI-04-C

Signed-off-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>