can: flexcan: flexcan_read_reg_iflag_rx(): optimize reading

The flexcan IP core has up to 64 mailboxes, each one has a corresponding
interrupt bit in the iflag1 or iflag2 registers and a mask bit in the
imask1 or imask2 registers.

In the timestamp (i.e. non FIFO) mode the driver needs to mask all non RX
interrupt sources, it uses the precomputed value rx_mask of struct flexcan_priv
for this.

In certain use cases, for example the CANFD mode, the contents of the iflag2
register is completely masked.

This patch optimizes the flexcan_read_reg_iflag_rx() function by not reading
the iflag1 or iflag2 register if the contents is masked.

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