usb: dwc3: Create helper function getting MDWIDTH

Different controller IPs check different HW parameters for MDWIDTH.
To help with maintainability, create a helper function to consolidate
the logic in a single place.

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/456329d36e8c188df5c234f3282595b630bf1470.1616892233.git.Thinh.Nguyen@synopsys.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>