bnx: Optimize RX STD ring refilling scheduling

bnx: Optimize RX STD ring refilling scheduling

At the end of the RX RET ring interrupt, the RX STD ring refilling is
scheduled only when the number of consumed RX BDs are more then half of
the RX STD ring size (256).

This reduces the IPI rate on CPU that performs the RX STD ring refilling
from ~17000ipis/s to ~4000ipis/s on the system with heavy network loading.