iommu/riscv: Fix signedness bug

The function platform_irq_count() returns negative error codes and
iommu->irqs_count is an unsigned integer, so the check
(iommu->irqs_count <= 0) is always impossible.

Make the return value of platform_irq_count() be assigned to ret, check
for error, and then assign iommu->irqs_count to ret.

Detected by Smatch:
drivers/iommu/riscv/iommu-platform.c:119 riscv_iommu_platform_probe() warn:
'iommu->irqs_count' unsigned <= 0

Signed-off-by: Ethan Tidmore <ethantidmore06@gmail.com>
Fixes: 5c0ebbd3c6c6 ("iommu/riscv: Add RISC-V IOMMU platform device driver")
Reviewed-by: Andrew Jones <andrew.jones@oss.qualcomm.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>