tools: Fix strncat() usage in hciconfig

According to strncat() manpage:

"If src contains n or more characters, strncat() writes n+1 characters
to dest (n from src plus the terminating null byte).  Therefore, the
size of dest must be at least strlen(dest)+n+1"

While the current usage of strncat() cannot cause an overflow, if a
bigger string is added to the switch()'s without increasing the static
buffer, it would overflow by one byte due to the incorrect size
calculation.