idpf: check error for register_netdev() on init

Current init logic ignores the error code from register_netdev(),
which will cause WARN_ON() on attempt to unregister it, if there was one,
and there is no info for the user that the creation of the netdev failed.

WARNING: CPU: 89 PID: 6902 at net/core/dev.c:11512 unregister_netdevice_many_notify+0x211/0x1a10
...
[ 3707.563641]  unregister_netdev+0x1c/0x30
[ 3707.563656]  idpf_vport_dealloc+0x5cf/0xce0 [idpf]
[ 3707.563684]  idpf_deinit_task+0xef/0x160 [idpf]
[ 3707.563712]  idpf_vc_core_deinit+0x84/0x320 [idpf]
[ 3707.563739]  idpf_remove+0xbf/0x780 [idpf]
[ 3707.563769]  pci_device_remove+0xab/0x1e0
[ 3707.563786]  device_release_driver_internal+0x371/0x530
[ 3707.563803]  driver_detach+0xbf/0x180
[ 3707.563816]  bus_remove_driver+0x11b/0x2a0
[ 3707.563829]  pci_unregister_driver+0x2a/0x250

Introduce an error check and log the vport number and error code.
On removal make sure to check VPORT_REG_NETDEV flag prior to calling
unregister and free on the netdev.

Add local variables for idx, vport_config and netdev for readability.

Fixes: 0fe45467 ("idpf: add create vport and netdev configuration")
Suggested-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Samuel Salin <Samuel.salin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>