net: ipa: add a clock reference for netdev operations

The IPA network device can be opened at any time, and an opened
network device can be stopped any time.  Both of these callback
functions require access to the hardware, and therefore they need
the IPA clock to be operational.  Take an IPA clock reference in
both the ->open and ->stop callback functions, dropping the
reference when they are done accessing hardware.

The ->start_xmit callback requires a little different handling,
and that will be added separately.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>