[WiFi] Fix NO_SIGNAL_STRENGTH value
[WiFi] Fix NO_SIGNAL_STRENGTH value

The signal strength is stored as an int16_t, so the existing
value 0xFFFF is out-of-bounds.

Using INT16_MAX instead, so 0x7FFF.

Resolves: LE-11297
Change-Id: Iea5714046be8217a41860fa3d91cebe5995739f5

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