nfp: bpf: don't offload XDP programs in DRV_MODE

DRV_MODE means that user space wants the program to be run in
the driver.  Do not try to offload.  Only offload if no mode
flags have been specified.

Remember what the mode is when the program is installed and refuse
new setup requests if there is already a program loaded in a
different mode.  This should leave it open for us to implement
simultaneous loading of two programs - one in the drv path and
another to the NIC later.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>