Fix common misspellings

Fixes generated by 'codespell' and manually reviewed.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
diff --git a/arch/powerpc/kernel/btext.c b/arch/powerpc/kernel/btext.c
index 625942a..60b3e37 100644
--- a/arch/powerpc/kernel/btext.c
+++ b/arch/powerpc/kernel/btext.c
@@ -99,7 +99,7 @@
 
 /* This function can be used to enable the early boot text when doing
  * OF booting or within bootx init. It must be followed by a btext_unmap()
- * call before the logical address becomes unuseable
+ * call before the logical address becomes unusable
  */
 void __init btext_setup_display(int width, int height, int depth, int pitch,
 				unsigned long address)