[ec/ecp_nistp*.c] remove flip_endian()

Replace flip_endian() by using the little endian specific
BN_bn2lebinpad() and BN_lebin2bn().

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/9511)

(cherry picked from commit e0b660c27d8d97b4ad9e2098cc957de26872c0ef)