nvme: fix I/O queue length calculation overflow

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>