bcusdk: eibd: drop local clock_gettime in USB backends

clock_gettime is defined locally, and calls pth_int_time, which
in turn calls clock_gettime.
The USB backend shouldn't overrule clock_gettime in the first place.
This patch fixes this endless recursion by removing the local defition.

Signed-off-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>