Bluetooth-OBEX: Reduce the MTU size if default OBEX MTU > 8k

Add a check for default OBEX MTU. If MTU is greater than 8k and
setMTU flag is true then reduce the MTU to 8k.

CRs-Fixed: 613931
Change-Id: I5785fd372241f682427ea3aca0b56ac747820d52