staging: mt7621-dma: sizeof via pointer dereference

Pass the size of a struct into kzalloc by pointer dereference.  This
complies with the Linux kernel coding style and removes the possibility
for a bug if the pointer's type is changed.

Signed-off-by: Benjamin Sherman <benjamin@bensherman.io>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>