bnx2x: refactor nvram read procedure

introduce a procedure to read in u32 granularity.

CC: Francious Romieu <romieu@fz.zoreil.com>
Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>