serial: sprd: Fix a copy-paste err in sprd_request_dma()

when dma_request_chan rx_dma fails, dev_err log should print rx_dma
err log.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>