tests: Bluetooth: Fix use of BTP PAST event in BSIM

The is_valid_gap_packet_len check for
btp_gap_ev_periodic_transfer_received_ev referenced a field
that was recently removed.

Modify the check to just check the size, as the event now
has not variable length fields.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>