hwrng: intel - cleanup initialization

clang static analysis reports this problem

intel-rng.c:333:2: warning: Assigned value is garbage or undefined
        void __iomem *mem = mem;
        ^~~~~~~~~~~~~~~~~   ~~~

Because mem is assigned before it is used, this is not
a real problem.  But the initialization is strange and not
needed, so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>