bnx2: Remove the rx_offset field from the bnx2 structure.

The rx_offset field is set to a constant value and initialized
only once.  By replacing all references to the rx_offset field,
we can eliminate rx_offset from the bnx2 structure.  This will
save 4 bytes for every bnx2 instance.

[Added parentheses to the definition of BNX2_RX_OFFSET, as noted
by Ben Hutchings.]

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>