can: c_can: fix "BUG! echo_skb is occupied!" during transmit

This patch fixes an issue with transmit routine, which causes
"can_put_echo_skb: BUG! echo_skb is occupied!" message when
using "cansequence -p" on D_CAN controller.

In c_can driver, while transmitting packets tx_echo flag holds
the no of can frames put for transmission into the hardware.

As the comment above c_can_do_tx() indicates, if we find any packet
which is not transmitted then we should stop looking for more.
In the current implementation this is not taken care of causing the
said message.

Also, fix the condition used to find if the packet is transmitted
or not. Current code skips the first tx message object and ends up
checking one extra invalid object.

While at it, fix the comment on top of c_can_do_tx() to use the
terminology "packet" instead of "package" since it is more
standard.

Cc: stable@kernel.org # 2.6.39+
Signed-off-by: AnilKumar Ch <anilkumar@ti.com>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>