iso: Fix handling same sequence number

When calculating the sequence number it may happen that we
compute the same value as for previous packet. If that happens
we manually increment the value by one. This was a problem when
yet another packet comes in a short interval and calculated seq.
number is still the same. Our calcualted seq. number for current
packet can now be lower than the previos one (after the previous
one was manually incremented). This resulted in wrongly calculated
packet lost counter dropping below 0.

Tag: #feature
Bug: 159786353
Sponsor: jpawlowski@
Test: atest --host net_test_btm_iso

Change-Id: I7d726ef292e85851af6383bfc8f4bcf125ccbf54