efi/cper: Fix cper_bits_to_str buffer handling and return value

commit d7f1b4b upstream.

The return value calculation was incorrect: `return len - buf_size;`
Initially `len = buf_size`, then `len` decreases with each operation.
This results in a negative return value on success.

Fix by returning `buf_size - len` which correctly calculates the actual
number of bytes written.

Fixes: a976d79 ("efi/cper: Add a new helper function to print bitmasks")
Signed-off-by: Morduan Zang <zhangdandan@uniontech.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Morduan Zang

drivers/firmware/efi/cper.c

@@ -162,7 +162,7 @@ int cper_bits_to_str(char *buf, int buf_size, unsigned long bits,
 		len -= size;
 		str += size;
 	}
-	return len - buf_size;
+	return buf_size - len;
 }
 EXPORT_SYMBOL_GPL(cper_bits_to_str);