nvme: fix I/O queue length calculation overflow

Commit cd47172 changed the I/O queue length calculation to use the
Maximum Queue Entries Supported (MQES) value from the capabilities
register, plus one, with a maximum value of NVME_PAGE_SIZE.

An unintended effect from this is that due to length being an unsigned
16-bit int, a MQES value of 0xFFFF yields a length of zero, resulting
in the queue allocation failing. Fix this by changing length to a u32.

TEST: build/boot on a Purism Librem13v2 with a MyDigitalSSD BPX NVMe
drive, which reports a MQES of 0xFFFF. Verify NVMe drive present in
boot menu and OS boots successfully.

Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
diff --git a/src/hw/nvme.c b/src/hw/nvme.c
index e6d739d..2e3aa38 100644
--- a/src/hw/nvme.c
+++ b/src/hw/nvme.c
@@ -318,7 +318,7 @@
 {
     int rc;
     struct nvme_sqe *cmd_create_cq;
-    u16 length = 1 + (ctrl->reg->cap & 0xffff);
+    u32 length = 1 + (ctrl->reg->cap & 0xffff);
     if (length > NVME_PAGE_SIZE / sizeof(struct nvme_cqe))
         length = NVME_PAGE_SIZE / sizeof(struct nvme_cqe);
 
@@ -362,7 +362,7 @@
 {
     int rc;
     struct nvme_sqe *cmd_create_sq;
-    u16 length = 1 + (ctrl->reg->cap & 0xffff);
+    u32 length = 1 + (ctrl->reg->cap & 0xffff);
     if (length > NVME_PAGE_SIZE / sizeof(struct nvme_cqe))
         length = NVME_PAGE_SIZE / sizeof(struct nvme_cqe);