nvme-pci: check kstrtoint() return value in queue_count_set()

This patch avoids that the compiler complains about 'ret' being set
but not being used when building with W=1.

Fixes: 3b6592f7 ("nvme: utilize two queue maps, one for reads and one for writes") # v5.0-rc1
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>