drivers: fix dma_get_required_mask

There's a bug in the current implementation of dma_get_required_mask()
where it ands the returned mask with the current device mask.  This
rather defeats the purpose if you're using the call to determine what
your mask should be (since you will at that time have the default
DMA_32BIT_MASK).  This bug results in any driver that uses this function
*always* getting a 32 bit mask, which is wrong.

Fix by removing the and with dev->dma_mask.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>