bn/bn_exp.c: harmonize all code paths with last commit.

848113a3 added mitigation for a
side-channel attack. This commit extends approach to all code
paths for consistency.

[It also removes redundant white spaces introduced in last commit.]

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6480)