staging: wilc1000: fixes a struct allocation to match coding standards

This patch fixes the checks reported by checkpatch.pl
for prefer kmalloc(sizeof(*tx_data)...) over kmalloc(sizeof(struct tx_complete_data)...)

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