libbladeRF: Added gain hotkey support to repeater test program

The numeric keys may be used to increase/decrease gains while running.
libbladeRF: Minor touch-ups in test_repeater

No need to use dummy variable with bladerf_init_stream as of commit
3e89a44f66239dfeb01ca6c5642e8c87c38cbbe7.

Reduced default sample rate to 1 Msps to avoid timeouts on Hi-speed
connections.