dsa: ptp: mark dummy helpers as 'inline'

Declaring a static function in a header leads to a warning every
time that header gets included without the function being used:

In file included from drivers/net/dsa/mv88e6xxx/chip.c:42:
drivers/net/dsa/mv88e6xxx/ptp.h:92:13: error: 'mv88e6xxx_hwtstamp_work' defined but not used [-Werror=unused-function]
 static long mv88e6xxx_hwtstamp_work(struct ptp_clock_info *ptp)
In file included from drivers/net/dsa/mv88e6xxx/chip.c:38:
drivers/net/dsa/mv88e6xxx/global2.h:355:12: error: 'mv88e6xxx_g2_wait' defined but not used [-Werror=unused-function]
 static int mv88e6xxx_g2_wait(struct mv88e6xxx_chip *chip, int reg, u16 mask)
            ^~~~~~~~~~~~~~~~~
drivers/net/dsa/mv88e6xxx/global2.h:350:12: error: 'mv88e6xxx_g2_update' defined but not used [-Werror=unused-function]
 static int mv88e6xxx_g2_update(struct mv88e6xxx_chip *chip, int reg, u16 update)
            ^~~~~~~~~~~~~~~~~~~
drivers/net/dsa/mv88e6xxx/global2.h:345:12: error: 'mv88e6xxx_g2_write' defined but not used [-Werror=unused-function]
 static int mv88e6xxx_g2_write(struct mv88e6xxx_chip *chip, int reg, u16 val)
            ^~~~~~~~~~~~~~~~~~
drivers/net/dsa/mv88e6xxx/global2.h:340:12: error: 'mv88e6xxx_g2_read' defined but not used [-Werror=unused-function]
 static int mv88e6xxx_g2_read(struct mv88e6xxx_chip *chip, int reg, u16 *val)

This marks all such functions in dsa inline to make sure we don't warn
about them.

Fixes: c6fe0ad2 ("net: dsa: mv88e6xxx: add rx/tx timestamping support")
Fixes: 0d632c3d ("net: dsa: mv88e6xxx: add accessors for PTP/TAI registers")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>