usb/hcd-ehci: Remove redundant statements

usb/hcd-ehci: Remove redundant statements

The "again" assignment is meaningless before g_assert_not_reached.
In addition, the break statements no longer needs to be after
g_assert_not_reached.

Clang static code analyzer show warning:
hw/usb/hcd-ehci.c:2108:13: warning: Value stored to 'again' is never read
            again = -1;
            ^       ~~

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200226084647.20636-13-kuhn.chenqun@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>