can: xilinx_can: Fix kernel doc warnings

This patch fixes below kernel doc warnings
warning: Function parameter or member 'priv' not described in
'xcan_write_frame'
warning: Function parameter or member 'skb' not described in
'xcan_start_xmit_fifo'
warning: Function parameter or member 'ndev' not described in
'xcan_start_xmit_fifo'
warning: Function parameter or member 'skb' not described in
'xcan_start_xmit_mailbox'
warning: Function parameter or member 'ndev' not described in
'xcan_start_xmit_mailbox'
warning: Function parameter or member 'priv' not described in
'xcan_rx_fifo_get_next_frame'

Signed-off-by: Appana Durga Kedareswara rao <appana.durga.rao@xilinx.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>