staging: dwc2: rename dwc2_check_core_status()

Rename dwc2_check_core_status() to dwc2_is_controller_alive(), and
make it a boolean function. Also change the message when the
controller is dead to say "dead" instead of "disconnected".

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>