src/mainboard: Fix various typos

These typos were found through manual review and grep.

Change-Id: Ia5a9acae4fbe2627017743106d9326a14c99a225
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/22525
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/mainboard/google/reef/mainboard.c

@@ -72,7 +72,7 @@ uint8_t __attribute__((weak)) variant_board_sku(void)
 	return board_sku_num;
 }
 
-/* Set variabnt board sku to ec by sku id */
+/* Set variant board sku to ec by sku id */
 void __attribute__((weak)) variant_board_ec_set_skuid(void)
 {
 }