iwlwifi: Fix -EIO error code that is never returned

iwlwifi: Fix -EIO error code that is never returned

Currently the error -EIO is being assinged to variable ret when
the READY_BIT is not set but the function iwlagn_mac_start returns
0 rather than ret. Fix this by returning ret instead of 0.

Addresses-Coverity: ("Unused value")
Fixes: 7335613ae27a ("iwlwifi: move all mac80211 related functions to one place")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210907104658.14706-1-colin.king@canonical.com
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>