ice: add support for reading and unpacking Rx queue context

In order to support live migration, the ice driver will need to read
certain data from the Rx queue context. This is stored in the hardware in a
packed format.

Since we use <linux/packing.h> for the mapping between the packed hardware
format and the unpacked structure, it is trivial to enable unpacking
support via the unpack_fields() function.

Add the ice_unpack_rxq_ctx() function based on the unpack_fields() API.
Re-use the same field definitions from the packing implementation.

Add ice_copy_rxq_ctx_from_hw() to copy the Rx queue context data from the
hardware registers.

Use these to implement ice_read_rxq_ctx() which will return the Rx queue
context to the caller in its unpacked ice_rlan_ctx struct.

This will enable the migration logic access to the relevant data about the
Rx device queues. It can easily be copied to the target system as part of
the migration payload, where it will be used to configure the Rx queues.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Madhu Chittim <madhu.chittim@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>