bnx2x: Refactor bnx2x_sp_post().

Some of the SPQ (slow-path queue) operations will be used
by the cnic code in later patches.

Signed-off-by: Shmulik Ravid - Rabinovitz <shmulikr@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Acked-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>