Bluetooth: L2CAP: Use fixed size for net_buf user data

The "sent" tracking doesn't have to be a signed integer. Use a fixed
size so that the consumed size doesn't change between different
architectures. Use a u16_t since bigger sizes are needed and because
this is mapped to an int function return higher up in the stack.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>