usb: dwc_otg: Don't use dma_to_virt()

Commit 6ce0d200 changes dma_to_virt() which breaks this driver.
Open code the old dma_to_virt() implementation to work around this.

Limit the use of __bus_to_virt() to cases where transfer_buffer_length
is set and transfer_buffer is not set. This is done to increase the
chance that this driver will also work on ARCH_BCM2835.

transfer_buffer should not be NULL if the length is set, but the
comment in the code indicates that there are situations where this
might happen. drivers/usb/isp1760/isp1760-hcd.c also has a similar
comment pointing to a possible: 'usb storage / SCSI bug'.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>