mptfusion: make adapter prod_name[] a pointer

The struct _MPT_ADAPTER doesn't need a full copy of the product string,
so prod_name can point to the string literal storage that the driver
already provides.

Avoids the following smatch warning:

  drivers/message/fusion/mptbase.c:2858 MptDisplayIocCapabilities()
    warn: this array is probably non-NULL. 'ioc->prod_name'

Signed-off-by: Joe Lawrence <joe.lawrence@stratus.com>
Acked-by: Sreekanth Reddy <Sreekanth.Reddy@avagotech.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>