virtio_blk: allow 0 as num_request_queues

The default value is 0 meaning "no limit". However if 0
is specified on the command line it is instead silently
converted to 1. Further, the value is already validated
at point of use, there's no point in duplicating code
validating the value when it is set.

Simplify the code while making the behaviour more consistent
by using plain module_param.

Fixes: 1a662cf6cb9a ("virtio-blk: add num_request_queues module parameter")
Cc: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>