OvmfPkg/RiscVVirt: VirtNorFlashPlatformLib: Fix wrong flash size

The size should be for single region, not the whole firmware FD.

Signed-off-by: Tuan Phan <tphan@ventanamicro.com>
Reviewed-by: Andrei Warkentin <andrei.warkentin@intel.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>

OvmfPkg/RiscVVirt/Library/VirtNorFlashPlatformLib/VirtNorFlashStaticLib.c

@@ -24,7 +24,8 @@ VIRT_NOR_FLASH_DESCRIPTION  mNorFlashDevice =
 {
   FixedPcdGet32 (PcdOvmfFdBaseAddress),
   FixedPcdGet64 (PcdFlashNvStorageVariableBase),
-  FixedPcdGet32 (PcdOvmfFirmwareFdSize),
+  FixedPcdGet32 (PcdOvmfFirmwareFdSize) -
+  (FixedPcdGet64 (PcdFlashNvStorageVariableBase) - FixedPcdGet32 (PcdOvmfFdBaseAddress)),
   QEMU_NOR_BLOCK_SIZE
 };