greybus: rename struct greybus_device

The greybus_device structure represents an Ara phone module.
It does *not* (necessarily) represent a UniPro device, nor any
device (like an i2c adapter) that might reside on an Ara module.

As such, rename struct greybus_device to be struct greybus_module.
Rename all symbols having that type to be "gmod" rather than "gdev".

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