staging: rtl8712u: Fix compiler warning about strncpy

When strncpy() is called with source and destination strings the same
length, gcc 8 warns that there may be an unterminated string. Using
strlcpy() rather than strncpy() forces a null at the end and quiets the
warning.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>