char: amd64-agp: Use 64-bit arithmetic instead of 32-bit

Cast *tmp* and *nb_base* to u64 in order to give the compiler
complete information about the proper arithmetic to use.

Notice that such variables are used in contexts that expect
expressions of type u64 (64 bits, unsigned) and the following
expressions are currently being evaluated using 32-bit arithmetic:

tmp << 25
nb_base << 25

Addresses-Coverity-ID: 200586 ("Unintentional integer overflow")
Addresses-Coverity-ID: 200587 ("Unintentional integer overflow")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>