atmel: hide unused procfs helpers

When CONFIG_PROC_FS isn't set, gcc warning this:

drivers/net/wireless/atmel/atmel.c:1402:12: warning: ‘atmel_proc_show’ defined but not used [-Wunused-function]
 static int atmel_proc_show(struct seq_file *m, void *v)
            ^
fix this by adding #ifdef around it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>