memstick: make enable_dma less generic in r592

Fixes this build error:

  drivers/memstick/host/r592.c:26: error: 'enable_dma' redeclared as different kind of symbol
  arch/powerpc/include/asm/dma.h:189: note: previous definition of 'enable_dma' was here

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Alex Dubov <oakad@yahoo.com>
Acked-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>