Fix common misspellings

Fixes generated by 'codespell' and manually reviewed.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
diff --git a/arch/sh/kernel/irq.c b/arch/sh/kernel/irq.c
index 64ea0b1..9197110 100644
--- a/arch/sh/kernel/irq.c
+++ b/arch/sh/kernel/irq.c
@@ -183,7 +183,7 @@
 		);
 
 		/*
-		 * Shouldnt happen, we returned above if in_interrupt():
+		 * Shouldn't happen, we returned above if in_interrupt():
 		 */
 		WARN_ON_ONCE(softirq_count());
 	}