net: dsa: mv88e6xxx: Fix -Wformat-security warnings

We are not specifying an explicit format argument but instead passing a
string litteral which causes these two warnings to show up:

drivers/net/dsa/mv88e6xxx/chip.c: In function
'mv88e6xxx_irq_poll_setup':
drivers/net/dsa/mv88e6xxx/chip.c:483:2: warning: format not a string
literal and no format arguments [-Wformat-security]
  chip->kworker = kthread_create_worker(0, dev_name(chip->dev));
  ^~~~
drivers/net/dsa/mv88e6xxx/ptp.c: In function 'mv88e6xxx_ptp_setup':
drivers/net/dsa/mv88e6xxx/ptp.c:403:4: warning: format not a string
literal and no format arguments [-Wformat-security]
    dev_name(chip->dev));
    ^~~~~~~~
  LD [M]  drivers/net/dsa/mv88e6xxx/mv88e6xxx.o

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>