Bluetooth: Create function to return the ERTM header size

Simplify the handling of different ERTM header size. We were the same
check in some places of the code, and more is expected to come, so just
replace them with a function.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Acked-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>