greybus: remove stray nul byte in apb_log_enable_read output

Including a nul byte in the otherwise human-readable ascii output
from this debugfs file is probably not intended.

Acked-by: Alex Elder <elder@linaro.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20210326152254.733066-1-linux@rasmusvillemoes.dk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>