staging: dwc2: remove specific fifo size constants

A generic set of FIFOSIZE_* constants is defined which applies to all
fifo size and offset registers. It is already used for both the
GNPTXFSIZ and HPTXFSIZ registers, but it applies to DPTXFSIZN as well.
Some of these also had specific constants defined. This patch removes
the specific constants and documents to use the generic constants.

Note that the removed constants weren't actually used. Instead, most of
the related code uses hardcoded masks and shifts. But given that
subsequent patches will be moving that code around and introducing the
constants in the process, this patch leaves those untouched.

Also note that the GRXFSIZ register also contains a fifo size, but
there is no corresponding start address register (it is always the first
fifo in memory), the layout of the GRXFSIZ register is different and
cannot use the same constants.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>