Fix Windows builds, which were getting "possible loss of data"

"bignum_new.c(61,52): warning C4244: 'function': conversion from 'mbedtls_mpi_uint' to 'unsigned int', possible loss of data"

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/bignum_new.c b/library/bignum_new.c
index 1c7523e..024348a 100644
--- a/library/bignum_new.c
+++ b/library/bignum_new.c
@@ -49,12 +49,11 @@
         (void) mbedtls_mpi_core_mla( T, n + 2, N, n, u1 );
     }
 
-    mbedtls_mpi_uint carry, borrow, fixup;
+    mbedtls_mpi_uint carry, borrow;
 
     carry  = T[n];
     borrow = mbedtls_mpi_core_sub( X, T, N, n );
-    fixup  = carry < borrow;
-    (void) mbedtls_mpi_core_add_if( X, N, n, fixup );
+    (void) mbedtls_mpi_core_add_if( X, N, n, ( carry < borrow ) );
 }
 
 mbedtls_mpi_uint mbedtls_mpi_core_mla( mbedtls_mpi_uint *d, size_t d_len,