bnx2x: Fix namespace collision with FLOW_CTRL_{TX,RX}

These are now defined in linux/mii.h and the bnx2x driver
defines different values which are shared with hardware
data structures.

So add a "BNX2X_" prefix to these macro names.

Based upon a report from Stephen Rothwell.

Signed-off-by: David S. Miller <davem@davemloft.net>