Bluetooth: l2cap: Decouple segmentation size

L2CAP Tx segmentation used BT_L2CAP_RX_MTU value which is
the value used by fixed channel protocols. Decoupling the
buffer size provides the opportunity to reduce RAM used per
connection.

Change-id: Id064f9b2e3f02073402815d09c3ea13a35df2a6c
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>