ath5k: constify stuff

Make some structures const to place them in .rodata, since we won't
change them.

Most important parts of objdump -h:
-  0 .text         00011170
+  0 .text         00011140
-  5 .rodata       0000828e
+  5 .rodata       0000895e
- 13 .data         00000560
+ 13 .data         00000110
- 14 .devinit.data 00000260

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis R. Rodriguez <lrodriguez@atheros.com>
Cc: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>