sd-hwdb: fix matching for characters with an ord > 127

sd-hwdb: fix matching for characters with an ord > 127

Devices like the "Microsoft Microsoft® 2.4GHz Transceiver v9.0 Mouse" contain
characters higher than 127. That ® is correctly stored in the hwdb and passed
into the search field during query, but the comparison fails.

Our search string is a const char *, trie_string() returns a const char * but
the current character is cast to uint8_t. This causes anything over 127 to
fail the match. Fix this, we're dealing with characters everywhere here after
all.