staging: gasket: avoid copy to user on error in coherent alloc config

gasket_config_coherent_allocator() on error return the error to caller
without copying a possibly-update DMA address back to userspace.

Reported-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>