bladeRF-cli: Fixed dangling FILE*

The following scenario was resulting in the assertion at line 235.
tx_csv_to_c16's interface specifies that it modiefies the file
descriptor -- in the failure case this means that it should close it
down.

```
bladeRF> tx config file=/dev/urandom format=csv

tx: Invalid I value encountered in CSV file

bladeRF> tx config format=bin
bladeRF> tx start
```