wlcore: remove stray plus sign

The commit mentioned below added a stray plus sign, likely
due to some conflict resolution (i.e. as a leftover from a
unified diff), which was harmless since it was just used as
an integer constant modifier. Remove it anyway, now that I
stumbled across it.

Fixes: cf33a772 ("wlcore: mesh: Add support for RX Broadcast Key")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>