crypto/dsa: change bitwise checks to mod operations
crypto/dsa: change bitwise checks to mod operations

Even though bitwise operations may be slightly more
performant, the readability improvement of a mod
operation is worth the tradeoff.

Change-Id: I352c92ad355c6eb6ef99e3da00e1eff2d2ea5812
Reviewed-on: https://go-review.googlesource.com/c/go/+/204739
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>
Run-TryBot: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

akutz/go,CAFxX/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,akutz/go,golang/go,golang/go,akutz/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,akutz/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,akutz/go,golang/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,golang/go,nawawi/go,golang/go,akutz/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go