staging: vt6656: Replace embedded function name with __func__ in rf.c

Change embedded function name in vnt_rf_set_txpower with %s format with
__func__ argument to make it consistent with other part of if-else and
kernel coding style standards as reported by checkpatch.

Signed-off-by: Daniel Cashman <dan.a.cashman@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>