iwlwifi: mvm: fix off-by-one in scan channels configuration

tail should be equal to the last valid index, so
decrease it by one.

This error causes in "a gap" in some cases (as well as
some possible out-of-bound write), finally resulting in
ucode assertion.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>