This patch avoids shifting the count left and right needlessly for each
call to sha1_update().  It instead can be done only once at the end in
sha1_final().

Keeping the previous test example (sha1_update() successively called with
len=64), a 1.3% performance increase can be observed on i386, or 0.2% on
ARM.  The generated code is also smaller on ARM.
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>