[BNX2]: Fix nvram write logic.

The nvram dword alignment logic was broken when writing less than 4
bytes on a non-aligned offset.  It was missing logic to round the
length to 4 bytes.

The page erase code is also moved so that it is only called when
using non-buffered flash for better code clarity.

Update version to 1.5.7.

Based on initial patch from Tony Cureington <tony.cureington@hp.com>.

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