spi: pic32: remove unnecessary of_node_get()

Almost all spi drivers assign spi master->dev.of_node from
its parent platform device without additional refcounting.
It seems of_node_get() in pic32_spi_probe() is unnecessary
and there is no corresponding of_node_put().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Mark Brown <broonie@kernel.org>