bnx2: Rename RX_COPY_THRESH to BNX2_RX_COPY_THRESH.

To make the bnx2 code more consistent, all instances of
RX_COPY_THRESH have been changed to BNX2_RX_COPY_THRESH.

Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>