siglent-sds: Replace non-portable strcasestr() with g_strstr_len().

siglent-sds: Replace non-portable strcasestr() with g_strstr_len().

The strcasestr() function is non-portable (e.g. not available on
MinGW, possibly elsewhere). Replace it with g_strstr_len() for the time
being. While the latter is not case-insensitive it appears that
that property doesn't matter here ("." should not be relevant anyway,
and e.g. an SDS1202X-E does indeed report "Mpts" as such).

Should it become necessary to have this code be case-insensitive,
we'll have to find a more portable solution than strcasestr().