staging: rtl8192e: Fix compiler warning from strncpy()

When strncpy() is called with source and destination strings the same
length, gcc 8 warns that there may be an unterminated string. This section
is completely reworked to use the known lengths of the strings.

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