can: c_can: Fix buffer ordering

The buffer handling of c_can has been broken forever. That leads to
message reordering:

ksoftirqd/0-3     [000] ..s.    79.123776: c_can_poll: rx_poll: val: 00007fff
ksoftirqd/0-3     [000] ..s.    79.124101: c_can_poll: rx_poll: val: 00008001

What happens is:

CPU				HW
				queue new packet into obj 16 (0-15 are busy)
read obj 1-15
return because pending is 0
				set pending obj 16 -> pending reg 8000
				queue new packet into obj 1
				set pending obj 1 -> pending reg 8001

So the current algorithmus reads the newest message first, which
violates the ordering rules of CAN.

Add proper handling of that situation by analyzing the contents of the
pending register for gaps.

This does NOT fix the message object corruption which can lead to
interrupt storms. Thats addressed in the next patches.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[mkl: adjusted subject]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>