Bluetooth: Host: Fix Advertising Coding Selection as peripheral

Fix mistake in host implementation of Advertising Coding Selection.

The host should only try set the BT_LE_FEAT_BIT_ADV_CODING_SEL_HOST
when the observer role is enabled. If a broadcaster enables the
CONFIG_BT_EXT_ADV_CODING_SELECTION Kconfig option bluetooth will
fail to initialise.

Signed-off-by: Thomas Deppe <thomas.deppe@nordicsemi.no>