gve: unify driver name usage

Current codebase contained the usage of two different names for this
driver (i.e., `gvnic` and `gve`), which is quite unfriendly for users
to use, especially when trying to bind or unbind the driver manually.
The corresponding kernel module is registered with the name of `gve`.
It's more reasonable to align the name of the driver with the module.

Fixes: 893ce44d ("gve: Add basic driver framework for Compute Engine Virtual NIC")
Cc: csully@google.com
Signed-off-by: Junfeng Guo <junfeng.guo@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>