efi/cper: Fix cper_bits_to_str buffer handling and return value

efi/cper: Fix cper_bits_to_str buffer handling and return value

commit d7f1b4bdc7108be1b178e1617b5f45c8918e88d7 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: a976d790f494 ("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>