Bluetooth: Only check SAR bits if frame is an I-frame

The SAR bits doesn't make sense for an S-frame. It doesn't use SAR.

Checking SAR for a S-frames can lead to L2CAP errors, it could close
the channel with an invalid packet length, since we was removing the 2
of the of any frame that match SAR start bits, without check if it is
an I-frame.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>