atmel: using strlcpy() to avoid possible buffer overflows

'firmware' is a module param which may been longer than firmware_id,
so using strlcpy() to guard against overflows. Also priv is allocated
with zeroed memory,no need to set firmware_id[0] to '\0'.

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