can: xilinx: Use proper fifo depths

The Axi CAN controller can be configured with different tx
and rx fifo depth. This patch add's support for this by reading
the hardware configured values from device-tree.Renamed the
variable xcan_echo_skb_max to xcan_echo_skb_max_tx.

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>