igb: Implement reading of reg RQDPC (Receive Queue Drop Packet Count)

Based on the previous patches from Jesper Dangaard Brouer <hawk@comx.dk>

Implement reading the per queue drop stats register
RQDPC (Receive Queue Drop Packet Count).  It counts the number of
packets dropped by a queue due to lack of descriptors available.

Notice RQDPC (Receive Queue Drop Packet Count) stats only gets
incremented, if the DROP_EN bit it set (in the SRRCTL register
for that queue).  If DROP_EN bit is NOT set, then the some what
equivalent count is stored in RNBC (not per queue basis).

The RQDPC register is only 12 bit, thus the precision might
suffer due to overrun in-netween the watchdog polling interval.

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
CC: Jesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>