Bignum: update const qualifiers

While at it, mark parameters based on their role.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/bignum_core.c b/library/bignum_core.c
index c015fc7..606765a 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -129,7 +129,7 @@
     return( mpi_bigendian_to_host_c( x ) );
 }
 
-void mbedtls_mpi_core_bigendian_to_host( mbedtls_mpi_uint * const X,
+void mbedtls_mpi_core_bigendian_to_host( mbedtls_mpi_uint *X,
                                          size_t limbs )
 {
     mbedtls_mpi_uint *cur_limb_left;