OvmfPkg/VirtNorFlashDxe: avoid array mode switch after each word write

NorFlashWriteSingleWord() switches into programming mode and back into
array mode for every single word that it writes. Under KVM, this
involves tearing down the read-only memslot, and setting it up again,
which is costly and unnecessary.

Instead, move the array mode switch into the callers, and only make the
switch when the writing is done.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>

OvmfPkg/VirtNorFlashDxe/VirtNorFlashDxe.c

@@ -280,6 +280,9 @@ NorFlashWriteFullBlock (
   }
 
 EXIT:
+  // Put device back into Read Array mode
+  SEND_NOR_COMMAND (Instance->DeviceBaseAddress, 0, P30_CMD_READ_ARRAY);
+
   if (!EfiAtRuntime ()) {
     // Interruptions can resume.
     gBS->RestoreTPL (OriginalTPL);