Fix type validation in the network create UAPI

Currently, the network create UAPI will assume that any network type
that isn't a buffer is an index. This means that the Linux kernel NPU
driver will accept any network type value and the user won't get any
feedback that they have specified an incorrect type.

To resolve this, the Linux kernel NPU driver will now return -EINVAL if
an unknown network type is given and a test has been added to validate
this behavior.

Change-Id: Ib7d9f5d5451897787981aae61a4e0a6650a73e05
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>