can: c_can: prepare to up the message objects number

As pointed by commit c0a9f4d3

 ("can: c_can: Reduce register
access") the "driver casts the 16 message objects in stone, which is
completely braindead as contemporary hardware has up to 128 message
objects".

The patch prepares the module to extend the number of message objects
beyond the 32 currently managed. This was achieved by transforming the
constants used to manage RX/TX messages into variables without
changing the driver policy.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20210302215435.18286-6-dariobin@libero.it
Signed-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>