As MCP251XFD_OBJ_FLAGS_DLC is a mask, add the missing _MASK postfix,
that all other masks in the driver have.

Link: https://lore.kernel.org/r/20210128104644.2982125-7-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>