can: isotp: padlen(): make const array static, makes object smaller

can: isotp: padlen(): make const array static, makes object smaller

Don't populate the const array plen on the stack but instead it static. Makes
the object code smaller by 926 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  26531	   1943	     64	  28538	   6f7a	net/can/isotp.o

After:
   text	   data	    bss	    dec	    hex	filename
  25509	   2039	     64	  27612	   6bdc	net/can/isotp.o

(gcc version 10.2.0)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20201020154203.54711-1-colin.king@canonical.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>