net: core: Fix kernel-doc for carrier_* attributes

Fix the documentation warning:

include/linux/netdevice.h:1939: warning: Excess struct member 'carrier_changes' description in 'net_device'

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: b2d3bcfa ("net: core: Expose number of link up/down transitions")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>