Bluetooth: shell: allow multiple parameters for bt hci-cmd

The hci-cmd currently accepts only a 1 byte parameter. This
disables use of commands that requires more than 1 byte,
for ex. the feature exchange that requires a 16 bit parameter.

This commit allows a datafield parameter up to 65 bytes long.
65 bytes comes from the nr. of parameters for the
HCI_LE_Generate_DHKey command

Signed-off-by: Andries Kruithof <Andries.Kruithof@nordicsemi.no>