mtd: mtdswap: make array 'name' static const, shrinks object size

Don't populate the read-only array 'name' on the stack but instead
make it static and constify it. Makes the object code smaller by 35
bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  26304	   4444	    352	  31100	   797c	drivers/mtd/mtdswap.o

After:
   text	   data	    bss	    dec	    hex	filename
  26205	   4508	    352	  31065	   7959	drivers/mtd/mtdswap.o

(gcc version 7.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>