[GNSS] Use gpsTimeValid for le_gnss_GetGpsTime()
[GNSS] Use gpsTimeValid for le_gnss_GetGpsTime()

Fix le_gnss_GetGpsTime() mistakenly use timeValid, use gpsTimeValid
instead.

Resolves: LE-11627
Change-Id: I0eae9c0441ed5bebfbbc9365cc2b49f644a0608d
Signed-off-by: Patrick Lai <7f880d785c948b1bb3e5e02f2825c319cfe59462@sierrawireless.com>

legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af