iwlwifi: fix double hyphen in MODULE_FIRMWARE for 8000

Mistakenly, the driver is trying to load the 8000C firmware with an
incorrect name (i.e. with two hyphens where there should be only one)
and that fails.  Fix that by removing the hyphen from the format
macro.

Fixes: e1ba684f ("iwlwifi: 8000: fix MODULE_FIRMWARE input")
Signed-off-by: Jürg Billeter <j@bitron.ch>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>