staging/mt7621-dma: mtk-hsdma.c->hsdma-mt7621.c

staging/mt7621-dma: mtk-hsdma.c->hsdma-mt7621.c

Also use KBUILD_MODNAME for module name.

This driver is only used by RALINK MIPS MT7621 SoCs. Tested by building
against that target using OpenWrt with Linux 5.10.10.

Fixes the following error:
error: the following would cause module name conflict:
  drivers/dma/mediatek/mtk-hsdma.ko
  drivers/staging/mt7621-dma/mtk-hsdma.ko

Cc: stable@vger.kernel.org
Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Link: https://lore.kernel.org/r/20210130034507.2115280-1-ilya.lipnitskiy@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>