staging: pi433: cleanup tx_fifo locking

pi433_write requires locking due to multiple writers.  After acquiring
the lock check if enough free space is available in the kfifo to write
the whole message. This check should prevent partial writes to tx_fifo
so kfifo_reset is not needed anymore.

pi433_tx_thread is the only reader so it does not require locking
after kfifo_reset is removed.

Signed-off-by: Valentin Vidic <Valentin.Vidic@CARNet.hr>
Reviewed-by: Marcus Wolf <marcus.wolf@wolf-entwicklungen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>