firmware: turris-mox-rwtm: fix building without CONFIG_KEYS

firmware: turris-mox-rwtm: fix building without CONFIG_KEYS

"struct key" is defined conditionally, so the code referencing it
must be made conditional as well:

In file included from drivers/firmware/turris-mox-rwtm.c:29:
include/linux/turris-signing-key.h: In function 'turris_signing_key_get_dev':
include/linux/turris-signing-key.h:26:19: error: invalid use of undefined type 'const struct key'
   26 |         return key->payload.data[1];
      |                   ^~

Signed-off-by: Arnd Bergmann <arnd@arndb.de>