net: ipv6: Cancel the prefix timer before setting it again
net: ipv6: Cancel the prefix timer before setting it again

If the earlier prefix timeout is not cancelled before setting
it again, the system will hang.

Change-Id: I6e271294c8e5d43e3ceae4780b5d1c26bab2b296
Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

fbsder/zephyr,fbsder/zephyr,fbsder/zephyr,fbsder/zephyr,fbsder/zephyr