3w-9xxx.c: Cleaning up missing null-terminate in conjunction with strncpy

Replacing strncpy with strlcpy to avoid strings that lacks null terminate.
And use the sizeof on the to string rather than strlen on the from string.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Acked-by: Adam Radford <aradford@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>