USB: remove obsolete config in kernel source (USB_HCD_DMA)

The configuration Option USB_HCD_DMA is not reachable in KConfig so
this piece of Code is effectively dead and useless. Remove it to avoid
confusion.

Signed-off-by: Christoph Egger <siccegge@stud.informatik.uni-erlangen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>