cryptsetup-generator: avoid magic value in ternary

`startswith` already returns the string with the prefix skipped, so we
can simplify this further and avoid using a magic value.

Noticed in passing.

Co-authored-by: Lennart Poettering <lennart@poettering.net>

src/cryptsetup/cryptsetup-generator.c

@@ -541,7 +541,7 @@ static int parse_proc_cmdline_item(const char *key, const char *value, void *dat
                 if (proc_cmdline_value_missing(key, value))
                         return 0;
 
-                d = get_crypto_device(startswith(value, "luks-") ? value+5 : value);
+                d = get_crypto_device(startswith(value, "luks-") ?: value);
                 if (!d)
                         return log_oom();