net: davicom: Fix regulator not turned off on driver removal

We must disable the regulator that was enabled in the probe function.

Fixes: 7994fe55

 ("dm9000: Add regulator and reset support to dm9000")
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: David S. Miller <davem@davemloft.net>