drivers: can: mcan: skip read/write to mram if data length is 0

can_mcan_send/can_mcan_get_message issued a message ram write/read even
when data length was 0. This caused some issues on systems where data
cache is enabled, because the cache invd/flushing calls present in the
mcan driver can fail if provided length is 0.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>