soundwire: qcom: use signed variable for error return

We get warning of using a unsigned variable being compared to less than
zero. The comparison is correct as it checks for errors from previous
call to qcom_swrm_get_alert_slave_dev_num(), so we should use a signed
variable here.

While at it, drop the superfluous initialization as well

drivers/soundwire/qcom.c: qcom_swrm_irq_handler() warn: impossible
condition '(devnum < 0) => (0-255 < 0)'

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210331155520.2987823-1-vkoul@kernel.org


Signed-off-by: Vinod Koul <vkoul@kernel.org>