staging: wfx: remove useless call to wfx_tx_flush()

wfx_do_join() calls wfx_tx_lock_flush() ate beginning of the function.
Therefore, the subsequent call to wfx_tx_flush() is useless.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200410133239.438347-8-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>