This patch refactors the code that is responsible to set the DMA mask for
the device.

Upon each change of the dma mask, the driver will save the new value that
was set. This is needed in order to make sure we don't try to increase the
mask a second time, in case we failed in the first time. This is
especially relevant for Power machines, as that may cause a change in
configuration of the TVT which will break the device.

Goya will first try to set the device's dma mask to 39 bits, so that the
memory that is allocated on the host machine for communication with the
device's cpu will be in a bus address which is lower then 39 bits. Later,
Goya will try to increase that mask to 48 bits, but only if setting the
mask to 39 bits was successful.
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>