Bluetooth-OPP: Revert Obex MTU size for SCO concurrency

This changes will revert OBEX MTU size back to maximum supported
MTU size for SCO concurrency. Otherwise, throughput will less than
excepted value.

Original change is to avoid OPP Tx failure during SCO concurrency due
to SoC limitation.

CRs-Fixed: 593686
Change-Id: I4f3f12e4dba34379458029b59e0e4d02b6db5598