Fix common misspellings

Fixes generated by 'codespell' and manually reviewed.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>

arch/sh/kernel/irq.c

@@ -183,7 +183,7 @@ asmlinkage void do_softirq(void)
 		);
 
 		/*
-		 * Shouldnt happen, we returned above if in_interrupt():
+		 * Shouldn't happen, we returned above if in_interrupt():
 		 */
 		WARN_ON_ONCE(softirq_count());
 	}