drivers: udc_dwc2: Suspend if connected to suspended bus

Peripherals are required to support the suspend state whenever VBUS
is powered, even if bus reset has not occurred (Mandate: Required,
Effective Date: February, 2010).

Remove the stale suspend check that essentially prevented the device
from hibernating when connected to already suspended bus.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>