staging: ralink-gdma: fix kconfig dependency bug for DMA_RALINK

When DMA_RALINK is enabled and DMADEVICES is disabled, it results in the
following Kbuild warnings:

WARNING: unmet direct dependencies detected for DMA_ENGINE
  Depends on [n]: DMADEVICES [=n]
  Selected by [y]:
  - DMA_RALINK [=y] && STAGING [=y] && RALINK [=y] && !SOC_RT288X [=n]

WARNING: unmet direct dependencies detected for DMA_VIRTUAL_CHANNELS
  Depends on [n]: DMADEVICES [=n]
  Selected by [y]:
  - DMA_RALINK [=y] && STAGING [=y] && RALINK [=y] && !SOC_RT288X [=n]

The reason is that DMA_RALINK selects DMA_ENGINE and DMA_VIRTUAL_CHANNELS
without depending on or selecting DMADEVICES while DMA_ENGINE and
DMA_VIRTUAL_CHANNELS are subordinate to DMADEVICES. This can also fail
building the kernel as demonstrated in a bug report.

Honor the kconfig dependency to remove unmet direct dependency warnings
and avoid any potential build failures.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=210055
Signed-off-by: Necip Fazil Yildiran <fazilyildiran@gmail.com>
Link: https://lore.kernel.org/r/20201104181522.43567-1-fazilyildiran@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>