ramfb: fix overflow

> CID 1393621:    (OVERFLOW_BEFORE_WIDEN)
> Potentially overflowing expression "stride * s->height" with type "unsigned
> int" (32 bits, unsigned) is evaluated using +32-bit arithmetic, and then used
> in a context that expects an expression of type "hwaddr" (64 bits, unsigned).

Fix by changing stride from uint32_t to hwaddr.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20180626083120.19515-1-kraxel@redhat.com