usb: cdns3: Optimize DMA request buffer allocation

dma_alloc_coherent() might fail on the platform with a small DMA region.

To avoid such failure in cdns3_prepare_aligned_request_buf(),
dma_alloc_coherent() is replaced with kmalloc and dma_map API to
allocate aligned request buffer of dynamic length.

Fixes: 7733f6c3

 ("usb: cdns3: Add Cadence USB3 DRD Driver")
Signed-off-by: Sanket Parmar <sparmar@cadence.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>