FmpDevicePkg/FmpDxe: Remove use of CatSprint()

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1525

The size overhead for CatSPrint() is large.  This function
is only used to generate variable names with HardwareInstance
value appended.  Use UnicodeValueToStringS() instead that is
much smaller.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Eric Jin <eric.jin@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>