can: xilinx_can: xcanfd_rx(): fix FSR register handling in the RX path

After commit c223da68 ("can: xilinx_can: Add support for CANFD FD
frames") the driver is updating the FSR IRI index multiple times (i.e in
xcanfd_rx() and xcan_rx_fifo_get_next_frame()), It should be updated
once per RX packet. This patch fixes this issue, also this patch removes
the unnecessary fsr register checks in xcanfd_rx() API.

Fixes: c223da68

 ("can: xilinx_can: Add support for CANFD FD frames")
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Reviewed-by: Shubhrajyoti Datta <Shubhrajyoti.datta@xilinx.com>
Signed-off-by: Appana Durga Kedareswara rao <appana.durga.rao@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>