Just have get_mac_lte_rapid_description() return the description string.

If get_mac_lte_rapid_description() returned FALSE, we always used "" as
the description string; just have it return the string, or return "" if
rapid ranges haven't been configured, and just use the string.

(If the code ever needs to determine in other places whether rapid ranges
have been configured, it can check s_rapid_ranges_configured, or
get_mac_lte_rapid_description() could return NULL in that case and the
code using the string can check for a null value.)

This should squelch some incorrect used-but-not-set warnings from at
least one compiler.

Change-Id: I3ed495a4beb4a553e453e7bca764d1d8a07c5e53
Reviewed-on: https://code.wireshark.org/review/4750
Reviewed-by: Guy Harris <guy@alum.mit.edu>