input: vl53l0x: copy more bytes when cat "show_meter" node

Measure.RangeMilliMeter has max length of 7 ,so increase
the copy length to make sure the data is copied entirely.

Change-Id: I708f6888b2694f8805aa6e11ba06add17d132c00
Signed-off-by: Chaojun Wang <chaojun@codeaurora.org>