floss: Add sysprop for LeGetVendorCapabilities

An existing controller incorrectly responds to LeGetVendorCapabilities
with the wrong OGF resulting in a stack crash. Add this sysprop to
allow temporarily disabling querying these capabilities until the
firmware is fixed.

Bug: 257423916
Tag: #floss
Test: Manual test on ChromeOS with sysprop set to false
Change-Id: I8c18ade9753f40693fd3e79ce1a72bdd7030a09c