cpufreq: interactive: fix compiling warnings

The gcc warns like:

  cpufreq_interactive.c:745:6: warning: operation on 'ret' may be
undefined [-Wsequence-point]

It was introduced by commit cf0fad49.

Since sprintf(...) just return 1 (one character) in this case, ret
should not changed.
Just discarding the result of sprintf(...) leads to the result that
the committer of cf0fad49 wants.

Change-Id: Ifed1cef6d6a31c3ed23dad03a567b3b9eddf3a57
Signed-off-by: Chih-Wei Huang <cwhuang@android-x86.org>
Signed-off-by: Dave Daynard <nardholio@gmail.com>