pcie-brcmstb: Eliminate arch_dma_ops error message

The driver attempts to set the dma_ops for the root complex, but doing
so causes an error message and only the end points need it. Fix the
error by making the code specific to the end point case.

Also copy some cosmetic tidy-ups from 5.5.y.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>