Bluetooth: tester: Fix write callback not returning errors properly

There is no need for any prepare write variables as this is now handled
transparently by the stack and so any errors that happens during write
shall be reported in the return.

Change-Id: I3e629016cb7f5f6cd9106ff07c3cbb464e81734f
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>