gpio: 104-dio-48e: make array 'ports' static, shrinks object size

Don't populate the const read-only array 'ports' on the stack but instead
make it static. Makes the object code smaller by over 100 buytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  10959	   4952	    832	  16743	   4167	drivers/gpio/gpio-104-dio-48e.o

After:
   text	   data	    bss	    dec	    hex	filename
  10790	   5008	    832	  16630	   40f6	drivers/gpio/gpio-104-dio-48e.o

(gcc version 7.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>