staging: wilc1000: rename index to tcp_pending_ack_idx

This patch renames "index" of struct txq_entry_t to tcp_pending_ack_idx
since this name could be confused index of txq_entry_t. It is index of
tcp pending ack.

It fixes 8e55639d ("staging: wilc1000: rename tcp_PendingAck_index
of struct txq_entry_t")

Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>