net: dsa: mv88e6xxx: move mv88e6xxx_hwtstamp_work() prototype

Since mv88e6xxx_hwtstamp_work() is defined in hwtstamp.c, its prototype
should be in hwtstamp.h, so move it there. Remove it's redundant stub
definition, as both hwtstamp.c (the function provider) and ptp.c (the
consumer) are both dependent on the same config symbol.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>