can: flexcan: flexcan_irq(): rename variable reg_iflag -> reg_iflag_rx

This patch renames the variable reg_iflag in the flexcan_irq() function
to reg_iflag_rx. This better reflects the contents of the varibale. It
does not hold the unmodified iflag registers, instead all non RX
interrupts have been masked.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>