iwlwifi: use IWL_DEBUG_SCAN for debug scanning

Replace IWL_DEBUG_{INFO,HC,RC} to IWL_DEBUG_SCAN in iwl-scan.c file. Add
some more IWL_DEBUG_SCAN messages. This will allow to fully debug
scanning using only IWL_DL_SCAN flag.

Also start one message sentence with capital letter, since that
convention in iwl-scan.c file.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>