OvmfPkg/PlatformBootManagerLib: Don't update progress if Pcd is 0

OvmfPkg/PlatformBootManagerLib: Don't update progress if Pcd is 0

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2266

Independently of how we decide to address other aspects of the regression
introduced with commit 2de1f611be06ded3a59726a4052a9039be7d459b, it doesn't
make much sense to call for a progress update if PcdPlatformBootTimeOut is
zero.

PcdPlatformBootTimeOut 0, which is the cause of the bug (division by zero)
should be considered to indicate that a platform is not interested in
displaying a progress report, so we alter PlatformBootManagerWaitCallback
to behave that way.

We also change one variable name to make the code more explicit.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Message-Id: <20191014150311.16740-2-pete@akeo.ie>