iwlwifi: pcie: fix some kernel-doc comments

"ubd" is really called "used_bd", fix that.

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.20210617100544.2d4b46c656bb.Iff9ee6a7e65d439169202911dad2cbea626fb887@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>