can: c_can: Speed up rx_poll function

This patch speeds up the rx_poll function by reducing the number of
register reads.

Replace the 32bit register read by a 16bit register read. Currently
the 32bit register read is implemented by using 2 16bit reads. This is
inefficient as we only use the lower 16bit in rx_poll.

The for loop reads the pending interrupts in every iteration. This
leads up to 16 reads of pending interrupts. The patch introduces a new
outer loop to read the pending interrupts as long as 'quota' is above 0.
This reduces the total number of reads.

The third change is to replace the for-loop by a ffs loop.

Tested on AM335x. I removed all 'static' and 'inline' from c_can.c to
see the timings for all functions. I used the function tracer with
trace_stats.

125kbit:
  Function                               Hit    Time            Avg             s^2
  --------                               ---    ----            ---             ---
  c_can_do_rx_poll                     63960    10168178 us     158.977 us      1493056 us
With patch:
  c_can_do_rx_poll                     63941    3764057 us      58.867 us       776162.2 us

1Mbit:
  Function                               Hit    Time            Avg             s^2
  --------                               ---    ----            ---             ---
  c_can_do_rx_poll                     69489    30049498 us     432.435 us      9271851 us
With patch:
  c_can_do_rx_poll                    207109    24322185 us     117.436 us      171469047 us

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>