net/sfc: support ops to check ready descriptors in ESSB Rx
net/sfc: support ops to check ready descriptors in ESSB Rx

Implement EF10 ESSB Rx datapath callback to get number of pending
descriptors.

Signed-off-by: Andrew Rybchenko <ac94ab2a8fe9a9a087f6e24dcbc16626b52c07e8@solarflare.com>
Reviewed-by: Ivan Malov <436a2feb05eb79a6862681e66bd0193220e8fb2d@oktetlabs.ru>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk