NTB: fix kernel-doc warnings in ntb.h

NTB: fix kernel-doc warnings in ntb.h

Correct a function name and function parameter name to avoid
kernel-doc warnings:

Warning: include/linux/ntb.h:575 expecting prototype for
 ntb_default_port_count(). Prototype was for ntb_default_peer_port_count()
 instead
Warning: include/linux/ntb.h:590 function parameter 'pidx' not
 described in 'ntb_default_peer_port_number'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>