lib/cmdline: Fix an invalid format specifier in an assertion msg

The correct format specifier for p - n (both p and n are pointers) is
%td, as the type should be ptrdiff_t.

This was discovered by annotating KUnit assertion macros with gcc's
printf specifier, but note that gcc incorrectly suggested a %d or %ld
specifier (depending on the pointer size of the architecture being
built).

Fixes: 0ea09083 ("lib/cmdline: Allow get_options() to take 0 to validate the input")
Signed-off-by: David Gow <davidgow@google.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>