greybus: loopback: fix 64-bit divisions

The code uses 64-bit divisions, which should be avoided, and also
prevents the module from loading on 32-bit systems:

	gb_loopback: Unknown symbol __aeabi_uldivmod (err 0)

Fix by using the kernel's 64-bit by 32-bit division implementation
do_div.

Compile tested only. I did not look very closely at the code itself.
Perhaps this could be worked around in some other way, but this silences
the linker warning and allows the module to be loaded.

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