Refine vendor subcmd QCA_NL80211_VENDOR_SUBCMD_ROAM_STATS

Refine vendor subcmd QCA_NL80211_VENDOR_SUBCMD_ROAM_STATS

During implementation of commit 257b119c2d87 ("QCA vendor attribute of
update roaming cached statistics info") some deficiencies were noted as
listed below, so fix them. Since these are pre-implementation changes,
no ABI breakage is introduced.

1) Change all RSSI values to be signed values.
2) Add enums for scan type and dwell type instead of documenting
   their values with comments
3) Add missing QCA_ROAM_REASON_STA_KICKOUT to enum qca_roam_reason

Signed-off-by: Chunquan Luo <quic_chunquan@quicinc.com>