Add kernel timestamp attribute in enum qca_wlan_vendor_attr_diag
for subcmd QCA_NL80211_VENDOR_SUBCMD_DIAG_DATA to send it from
driver to userspace. Also, QCA_WLAN_VENDOR_ATTR_DIAG_HOST_TIMESTAMP
attribute indicates the system timestamp but is documented as kernel
timestamp wrongly. So correct the documentation of this attribute to
reflect the correct behavior.
Change-Id: I9559345185cb75ae6adc6792126fc6679b089e5d
CRs-Fixed: 3150783