nfp: bpf: refactor nfp_bpf_check_ptr()

nfp_bpf_check_ptr() mostly looks at the pointer register.
Add a temporary variable to shorten the code.

While at it make sure we print error messages if translation
fails to help users identify the problem (to be carried in
ext_ack in due course).

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