[BNX2]: factor out gzip unpacker

This patch modifies gzip unpacking code in bnx2 driver so that
it does not depend on bnx2 internals. I will move this code
out of the driver and into zlib in follow-on patch.

It can be useful in other drivers which need to store firmwares
or any other relatively big binary blobs - fonts, cursor bitmaps,
whatever.

Patch is run tested by Michael Chan (driver author).

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Acked-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>