greybus: camera: only build module against msm kernel

The camera driver currently does not build against anything other than
the msm kernel, due to cross-dependancies, so enable that here so that
we can build against other kernels without failing the build.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Tested-by: Marti Bolivar <mbolivar@leaflabs.com>
Acked-by: Laurent Pinchart <laurent.pinchart@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>