nfp: split reading capabilities out of nfp_net_init()

nfp_net_init() is a little long and we are about to add more
code to reading capabilties.  Move the capability reading,
parsing and validating out.  Only actual initialization
will stay in nfp_net_init().

No functional changes.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>