adb: Fix return value in error case

When we introduced __adb_check_server_version() as a function
returning a 'bool', we missed missed converting once instance
of "return -1;" to "return false;".  Thus, we're returning 'true'
in this case as our non-zero value gets implicitly converted to
'true'.

We fix this case as well with this CL.

Test: TreeHugger
Change-Id: I90b01567d927f36d2d963561b676b3bfabfcee49