Try setting optional byte in set user acess

Set the user session limit explicitly to lift
any restrictions an implementation may default
to.  Some systems consider this byte mandatory
though the specification says optional

(From Steve Weber)

Change-Id: I95f4743ded702a436be019c902487813f916bd27

pyghmi/ipmi/command.py

@@ -705,7 +705,7 @@ class Command(object):
             'no_access': 0x0F,
         }
         data = [b, uid & 0b00111111,
-                privilege_levels[privilege_level] & 0b00001111]
+                privilege_levels[privilege_level] & 0b00001111, 0]
         response = self.raw_command(netfn=0x06, command=0x43, data=data)
         if 'error' in response:
             raise Exception(response['error'])