samples: hid: rework USB HID sample report and on_idle callback

HID sample has on_idle callback implementation present in the code,
but has not turned on USB_DEVICE_SOF and so does not have
corresponding code in sample self and class/hid/core.c covered.
Also on_idle implementation in HID sample has not taken into
account send_report() which also calls hid_int_ep_write().
This patch enables USB_DEVICE_SOF and revises both on_idle and
the regular report function.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>