net: wwan: qcom_bam_dmux: fix wrong pointer passed to IS_ERR()

It should check dmux->tx after calling dma_request_chan().

Fixes: 21a0ffd9

 ("net: wwan: Add Qualcomm BAM-DMUX WWAN network driver")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20220319032450.3288224-1-yangyingliang@huawei.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>