It's possible the AP could miss an incoming SVC timesync pulse i.e. the AP
could have interrupts switched off for long enough that one SVC GPIO strobe
ends up over-lapping another one. TimeSync should be able to deal with this
type of transitory failure by retrying a failed synchronous TimeSync resync
up to 'n' number of times. For this patch 'n' has been set to five, which
is a hand-wavy choice that 'feels' right.