net: ravb: Let IP-specific receive function to interrogate descriptors

ravb_poll() initial code used to interrogate the first descriptor of the
RX queue in case gPTP is false to determine if ravb_rx() should be called.
This is done for non-gPTP IPs. For gPTP IPs the driver PTP-specific
information was used to determine if receive function should be called. As
every IP has its own receive function that interrogates the RX descriptors
list in the same way the ravb_poll() was doing there is no need to double
check this in ravb_poll(). Removing the code from ravb_poll() leads to a
cleaner code.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>