nvme-fabrics: don't check for non-NULL module in nvmf_register_transport

THIS_MODULE evaluates to NULL when used from code built into the kernel,
thus breaking built-in transport modules.  Remove the bogus check.

Fixes: 0de5cd36 ("nvme-fabrics: protect against module unload during create_ctrl")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Keith Busch <keith.busch@intel.com>