can: only call can_stat_update with procfs

The change to leave out procfs support in CAN when CONFIG_PROC_FS
is not set was incomplete and leads to a build error:

net/built-in.o: In function `can_init':
:(.init.text+0x9858): undefined reference to `can_stat_update'
ERROR: "can_stat_update" [net/can/can.ko] undefined!

This tries a better approach, encapsulating all of the calls
within IS_ENABLED(), so we also leave out the timer function
from the object file.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: a20fadf8 ("can: build proc support only if CONFIG_PROC_FS is activated")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>