QLowEnergyConttroller - char/descriptor values (OS X/iOS)

While writing with response, Core Bluetooth does not
update characteristic/descriptor values in write notifications.
I have to 'cache' this values and report them back.
Also this patch fixes minor bugs in '-performNextWriteRequest':
while handling errors it's not enough just to call 'performNextWriteRequest:'
again, it must be in return statement or followed by 'return'.

Change-Id: I6e7ab7fe6f63ea84268bf6c7ea67e2939d524f91
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>