iwlwifi: dbg: mark a variable __maybe_unused

If CONFIG_IWLWIFI_DEBUGFS is not set, the variable is assigned
but not checked, resulting in a compiler warning. Suppress it,
we need the variable for the debugfs-enabled case.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200425130140.485f886f5a6c.I8a91c560c26cced33b15d8419caebb53a9abcc2d@changeid