greybus: battery: use feature tag rather than kernel version

Conditionally define a new symbol DRIVER_OWNS_PSY_STRUCT, which is
set in "kernel_ver.h" based on on the kernel version.  Use it to
distinguish code used for kernels that differ in whether a power
supply structure is owned by the driver, or by the power supply
core.

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