Bluetooth: controller: Fix empty_pkt_us_get for 2M phy.

An empty packet on 2M phy is one more byte compared to 1M phy because
of the preamble.
The empty packet is then 11 bytes, which takes 44 us to transmit.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>