crypto: pcbc - remove redundant assignment to nbytes

crypto: pcbc - remove redundant assignment to nbytes

The assignment to nbytes is redundant, the while loop needs
to just refer to the value in walk.nbytes and the value of
nbytes is being re-assigned inside the loop on both paths
of the following if-statement.  Remove redundant assignment.

Cleans up clang scan build warning:
warning: Although the value stored to 'nbytes' is used in
the enclosing expression, the value is never actually read
from 'nbytes' [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>