Use correct variable.

To check whether data_rate_or_mcs_index is set, check
saw_data_rate_or_mcs_index; we might see a value of 0 for that field
(it's a valid MCS index), so we can't use it as an out-of-band value
meaning "not set".

Change-Id: I75d7fdb4a90836538c82f56f2afb05c0603278a5
Reviewed-on: https://code.wireshark.org/review/4991
Reviewed-by: Guy Harris <guy@alum.mit.edu>