Bluetooth: shell: Fix 'bt per-adv-data'

ad_len parameter is not length of complete advertising data, but number
of bt_data structs passed to bt_le_per_adv_set_data - we pass only one.

Also memset target buffer before converting hex string so data shorter
than specified will be right-padded with zeroes instead of some
leftovers.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>