Bluetooth: Improve ERTM sequence number offset calculation

Instead of using modular division, the offset can be calculated using
only addition and subtraction.  The previous calculation did not work
as intended and was more difficult to understand, involving unsigned
integer underflow and a check for a negative value where one was not
possible.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>