iwlwifi: fix unused variable warning

In the case of disabled CONFIG_IWLWIFI_DEBUGFS option the compiler complains
about the unused variable 'img'. Fix this by moving the 'img' definition.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>