Bluetooth: Remove unused err var from l2cap_segment_sdu()

Trivial fix, let the code cleaner.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>