Use correct bit positions in DM_SHARED_CTRL register for writes.

Michael Planes recently encountered a 'KY-RS9600 USB-LAN converter', which
came with a driver CD containing a Linux driver. This driver turns out to
be a copy of dm9601.c with symbols renamed and my copyright stripped.
That aside, it did contain 1 functional change in dm_write_shared_word(),
and after checking the datasheet the original value was indeed wrong
(read versus write bits).

On Michaels HW, this change bumps receive speed from ~30KB/s to ~900KB/s.
On other devices the difference is less spectacular, but still significant
(~30%).
Reported-by: Michael Planes <michael.planes@free.fr>
CC: stable@kernel.org
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>