can: flexcan: rename struct flexcan_priv::reg_imask{1,2}_default to rx_mask{1,2}

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 out all
non RX interrupt sources and uses the precomputed values
reg_imask1_default and reg_imask2_default of struct flexcan_priv for
this.

However in the current driver the reg_imask{1,2}_default cannot be used
directly to get the pending RX interrupts. The TX interrupt is part of
these variables, so it needs to be masked out, too.

This is a preparation patch to clean up calculation of the pending RX
interrupts, it only renames the variables from

    reg_imask{1,2}_default

to

    rx_mask{1,2}

To better reflect their meaning after the complete conversion. This
change is done with the following sed command:

    sed -i -e "s/reg_imask\(1\|2\)_default/rx_mask\1/" drivers/net/can/flexcan.c

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