Draft: telit: support #BND 4G LTE expressed in HEX format

Some modems, like LE910C1-EUX, express the LTE part of #BND output in HEX format, but currently the MM parser for such case is limited to modems that have "ext_4g_bands".

This commit extends the expected format of the LTE part of #BND test command to HEX strings, then uses the mm_get_u64_from_hex_str as fallback if mm_get_u64_from_str fails.

Fixes #487 (closed)