Bluetooth: Update tx_send_head when sending ERTM data

Commit 94122bbe introduced a problem
where tx_send_head was not set to point to the first skb in the ERTM
transmit queue, which stalled data transmission.  This change sets
that pointer when transmission is not already in progress.

Reported-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>