can: flexcan: fix flexcan driver build for big endian on ARM and little endian on PowerPc

There is no reason to disallow building the driver on big-endian ARM kernels.
Furthermore, the current behavior is actually broken on little-endian PowerPC
as well.

The choice of register accessor functions must purely depend on the CPU
architecture, not which endianess the CPU is running on. Note that we nowadays
allow both big-endian ARM and little-endian PowerPC kernels.

With this patch applied, we will do the right thing in all four combinations.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>