scsi: Use _res and not res in MK_ERROR.

Some users of MK_ERROR pass in parameters from a variable called "res",
which is obviously not what they want to do, as that will use this "res"
and not theirs.

Spotted by Clang.

headers/private/drivers/scsi_periph.h

@@ -44,8 +44,8 @@ typedef struct err_res {
 } err_res;
 
 #define MK_ERROR( aaction, code ) ({ \
-	err_res res = {error_code: (code), action: (aaction) };	\
-	res;					\
+	err_res _res = {error_code: (code), action: (aaction) };	\
+	_res;					\
 })
 
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