Bluetooth: Host: Align return lines of bt shell helper function phy2str

As other helper functions were introduced for the LE Power Control
Request Feature (see CONFIG_BT_TRANSMIT_POWER_CONTROL), it was noticed
that the return statements of switch cases should be on the next line
generally. This will align phy2str with the rest in bt.c.

Signed-off-by: Kyra Lengfeld <kyra.lengfeld@nordicsemi.no>