ks8851: Fix freed memory reference in probe failure

If ks8851_probe fails, deregister the regulator devices,
which are part of the ks structure, before freeing the
network device rather than dereferencing a freed pointer.
The regulator devices need to be deregistered before the
network device is freed because the private pointer (ks)
is part of the ndev structure.

Change-Id: Id8e7b25e26dbae2c038f6f018931fc25efbf5cff
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>