bluetooth: shell: match cmd_conn_phy_update conditions

The condition for channel-map are CONFIG_BT_CENTRAL ||
CONFIG_BT_BROADCASTER, change the corresponding handler ifdef so that
it's included in the build if CONFIG_BT_BROADCASTER is enabled but
CONFIG_BT_CENTRAL is not.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>