Cast number of operations to uint in MBEDTLS_ECP_BUDGET

Context:
The macro `MBEDTLS_ECP_BUDGET()` is called before performing a
number of potentially time-consuming ECC operations. If restartable
ECC is enabled, it wraps a call to `mbedtls_ecp_check_budget()`
which in turn checks if the requested number of operations can be
performed without exceeding the maximum number of consecutive ECC
operations.

Issue:
The function `mbedtls_ecp_check_budget()` expects a the number
of requested operations to be given as a value of type `unsigned`,
while some calls of the wrapper macro `MBEDTLS_ECP_BUDGET()` use
expressions of type `size_t`.
This rightfully leads to warnings about implicit truncation
from `size_t` to `unsigned` on some compilers.

Fix:
This commit makes the truncation explicit by adding an explicit cast
to `unsigned` in the expansion of the `MBEDTLS_ECP_BUDGET()` macro.

Justification:
Functionally, the new version is equivalent to the previous code.
The warning about truncation can be discarded because, as can be
inferred from `ecp.h`, the number of requested operations is never
larger than 1000.

include/mbedtls/ecp.h

@@ -233,7 +233,9 @@ int mbedtls_ecp_check_budget( const mbedtls_ecp_group *grp,
                               unsigned ops );
 
 /* Utility macro for checking and updating ops budget */
-#define MBEDTLS_ECP_BUDGET( ops )   MBEDTLS_MPI_CHK( mbedtls_ecp_check_budget( grp, rs_ctx, ops ) );
+#define MBEDTLS_ECP_BUDGET( ops )   \
+    MBEDTLS_MPI_CHK( mbedtls_ecp_check_budget( grp, rs_ctx, \
+                                               (unsigned) (ops) ) );
 
 #else /* MBEDTLS_ECP_RESTARTABLE */