Add explicit type cast to avoid truncation warning

`mbedtls_rsa_deduce_primes` implicitly casts the result of a call to
`mbedtls_mpi_lsb` to a `uint16_t`. This is safe because of the size
of MPI's used in the library, but still may have compilers complain
about it. This commit makes the cast explicit.

library/rsa_internal.c

@@ -114,7 +114,7 @@ int mbedtls_rsa_deduce_primes( mbedtls_mpi const *N,
     MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &T, D,  E ) );
     MBEDTLS_MPI_CHK( mbedtls_mpi_sub_int( &T, &T, 1 ) );
 
-    if( ( order = mbedtls_mpi_lsb( &T ) ) == 0 )
+    if( ( order = (uint16_t) mbedtls_mpi_lsb( &T ) ) == 0 )
     {
         ret = MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
         goto cleanup;