usb: musb: Fix DMA desired mode for Mentor DMA engine

Commit 754fe4a9 ("usb: musb: Remove ifdefs for TX DMA for musb_host.c")
introduces a problem setting the desired channel mode for the Mentor DMA
engine.

There is a case where an address is incorrectly assigned to the DMA
channel desired mode when it should instead be assigned the actual mode
value. This results in the value of channel->desired_mode not being
correct.

Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Cristian Birsan <cristian.birsan@microchip.com>
Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>