net: stmmac: add __stmmac_release() to complement __stmmac_open()

Rename stmmac_release() to __stmmac_release(), providing a new
stmmac_release() method. Update stmmac_change_mtu() to use
__stmmac_release(). Move the runtime PM handling into stmmac_open()
and stmmac_release().

This avoids stmmac_change_mtu() needlessly fiddling with the runtime
PM state, and will allow future changes to remove code from
__stmmac_open() and __stmmac_release() that should only happen when
the net device is administratively brought up or down.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>