spi: pl022: Fix incorrect dma_unmap_sg

When mapped RX DMA entries are unmapped in an error condition when DMA
is firstly configured in the driver, the number of TX DMA entries was
passed in, which is incorrect

Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org