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>

drivers/soundwire/qcom.c

@@ -428,7 +428,7 @@ static irqreturn_t qcom_swrm_irq_handler(int irq, void *dev_id)
 	struct qcom_swrm_ctrl *swrm = dev_id;
 	u32 value, intr_sts, intr_sts_masked, slave_status;
 	u32 i;
-	u8 devnum = 0;
+	int devnum;
 	int ret = IRQ_HANDLED;
 
 	swrm->reg_read(swrm, SWRM_INTERRUPT_STATUS, &intr_sts);