Bluetooth: shell: Fix adv-data command hex data input

Fix adv-data command when given arbitrary advertising data in
hexadecimal format. The data_len field should contain the length of the
data which does not include the data type. Instead the AD len field in
the data was given. This caused the AD len field to be increased by 1
in the advertising dat.

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