net: dwc: Make the cache handling less cryptic

Add a few new variables to make the cache handling less cryptic.
Add a variable for DMA and DATA descriptor start and end, so the
correctness of the code is easier to inspect.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@altera.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Tom Rini <trini@ti.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Pavel Machek <pavel@denx.de>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Acked-by: Pavel Machek <pavel@denx.de>
Acked-by: Chin Liang See <clsee@altera.com>