net/nfp: fix lock file usage

net/nfp: fix lock file usage

DPDK apps can be executed as non-root users but current NFP lock
file for avoiding concurrent accesses to CPP interface is precluding
this option or requires to modify system file permissions.

When the NFP device is bound to VFIO, this driver does not allow this
concurrent access, so the lock file is not required at all.

OVS-DPDK as executed in RedHat distributions is the main NFP user
needing this fix.

Fixes: c7e9729da6b5 ("net/nfp: support CPP")
Signed-off-by: Alejandro Lucero <alejandro.lucero@netronome.com>