FmpDevicePkg/FmpDxe: Improve function parameter validation

FmpDevicePkg/FmpDxe: Improve function parameter validation

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2869

Makes some minor improvements to function parameter validation
in FmpDxe, in particular to externally exposed functions such
as those that back EFI_FIRMWARE_MANAGEMENT_PROTOCOL.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Guomin Jiang <guomin.jiang@intel.com>
Cc: Wei6 Xu <wei6.xu@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Guomin Jiang <guomin.jiang@intel.com>
Reviewed-by: Wei6 Xu <wei6.xu@intel.com>