spi: bcm2835aux: disable tx fifo empty irq

The tx empty irq can be disabled when all data was copied.
This prevents unnecessary interrupts while the last bytes are sent.

Signed-off-by: Stephan Olbrich <stephanolbrich@gmx.de>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Mark Brown <broonie@kernel.org>