can: xilinx_can: avoid non-requested bus error frames

Userspace can signal with CAN_CTRLMODE_BERR_REPORTING whether they need
reporting of bus errors (CAN_ERR_BUSERROR) or not.

However, xilinx_can driver currently always sends CAN_ERR_BUSERROR
frames to userspace on bus errors.

To improve performance on error conditions when bus error reporting is
not needed, avoid sending CAN_ERR_BUSERROR frames unless requested via
CAN_CTRLMODE_BERR_REPORTING.

The error interrupt is still kept enabled as there is no dedicated state
transition interrupt, but just disabling error frame submission still
yields a significant performance improvement. In a simple test with
continuous bus errors and no userspace programs reading/writing CAN I
saw system CPU load reduced by 1/3.

Tested on a ZynqMP board with CAN-FD v1.0.

Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

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