mv643xx_eth: switch ->phy_lock from a spinlock to a mutex

Since commit 81600eea ("mv643xx_eth:
use auto phy polling for configuring (R)(G)MII interface"),
mv643xx_eth no longer does SMI accesses from interrupt context.  The
only other callers that do SMI accesses all do them from process
context, which means we can switch the PHY lock from a spinlock to a
mutex, and get rid of the extra locking in some ethtool methods.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>