Staging: rtl8192u: fix checkpatch.pl error

This patch fixes the checkpatch.pl script error:
ERROR: do not use C99 // comments
+static long ieee80211_translate_todbm(u8 signal_strength_index)// 0-100
index.

Signed-off-by: Teodora Baluta <teobaluta@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>