greybus: loopback: use a 32-bit count

The count of statistical samples recorded is currently a 64-bit
value.  32 bits is sufficient, and in fact anything more than
that won't work for the do_div() call it's pass to anyway.  So make
the count field be 32 bits.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>