net: can: xilinx_can: fix extended frame handling

Using IDR_SRR in RXFIFO_ID to test for the presence of data is only
valid for standard frames. For extended frames the bit is always 1 and
IDR_RTR should be used instead. This patch switches the check to use
CAN_RTR_FLAG which is correctly set when reading the ID.

The patch also changes the DW1/DW2 to be read unconditionally, since
this is necessary to remove the frame from the RXFIFO.

Signed-off-by: Jeppe Ledet-Pedersen <jlp@gomspace.com>
Acked-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>