xen/public: Correct the definition of GNTTAB_CACHE_SOURCE_GREF

xen/public: Correct the definition of GNTTAB_CACHE_SOURCE_GREF

Discovered when running the XSA-232 PoC on a UBSAN-enabled hypervisor.

  (d79) XSA-232 PoC
  (XEN) ================================================================================
  (XEN) UBSAN: Undefined behaviour in grant_table.c:3217:25
  (XEN) left shift of 1 by 31 places cannot be represented in type 'int'
  (XEN) ----[ Xen-4.10.0-rc  x86_64  debug=y   Tainted:    H ]----

Update all of the GNTTAB_CACHE_* constants to be unsigned integers.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>