RPL MRHOF: bypass EWMA when setting a parent's link metric for the first time

Whenever getting a first neighbor callback, set the link metric to the current packet's ETX rather than using a weighted moving average with RPL_INIT_LINK_METRIC. Makes it faster to converge to an accurate link estimate. For instance, over a perfect link, with RPL_INIT_LINK_METRIC of 5 and an EWMA alpha of 0.9, it took about 50 transmissions to drop link_metric from 1280 (ETX of 5) to 256 (ETX of 1). With this fix, the first successful transmission sets link_metric to 256.