net: switchdev: change ageing_time type to clock_t

The switchdev value for the SWITCHDEV_ATTR_ID_BRIDGE_AGEING_TIME
attribute is a clock_t and requires to use helpers such as
clock_t_to_jiffies() to convert to milliseconds.

Change ageing_time type from u32 to clock_t to make it explicit.

Fixes: f55ac58a ("switchdev: add bridge ageing_time attribute")
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>