Bluetooth: Improve skb handling in mgmt_device_connected()

This patch introduce eir_skb_put_data() that can be used to simplify
operations on eir in goal of eliminating the necessity of intermediary
buffers.
eir_skb_put_data() is in pair to what eir_append_data() does with help of
eir_len, but without awkwardness when passing return value to skb_put() (as
it returns updated offset not size).

Signed-off-by: Radoslaw Biernacki <rad@semihalf.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>