PHY: remove rwsem use from phy core

The subsystem rwsem is not used by the driver core at all, so the use of
it in the phy code doesn't make any sense.  They might possibly
want to use a local lock, but I am unsure about that.

Cc: netdev <netdev@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>