Bluetooth: Fix ACL MTU issue

ERTM and Streaming Modes was having problems when the ACL MTU is lower
than MPS. The 'minus 10' is to take in account the header and fcs
lenghts.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>