watchdog: asm9260_wdt: make array mode_name static, shrinks object size

Don't populate the const array mode_name on the stack but instead
make it static. Makes the object code smaller by 41 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
   7699	   1872	      0	   9571	   2563	drivers/watchdog/asm9260_wdt.o

After:
   text	   data	    bss	    dec	    hex	filename
   7594	   1936	      0	   9530	   253a	drivers/watchdog/asm9260_wdt.o

(gcc version 8.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>