psa: Clear bits in mbedtls_ecc_group_to_psa()

Clear bits in mbedtls_ecc_group_to_psa() to avoid static analyzers and
possibly compilers from warning that bits may be used uninitialized in
certain code paths.

For example, if mbedtls_ecc_group_to_psa() were to be inlined in
crypto_extra.h, the following compiler warning is likely.

    In file included from ../include/psa/crypto.h:3774:0,
                     from ../include/mbedtls/pk.h:49,
                     from pk.c:29:
    pk.c: In function 'mbedtls_pk_wrap_as_opaque':
    ../include/psa/crypto_struct.h:460:33: error: 'bits' may be used uninitialized in this function [-Werror=maybe-uninitialized]
             attributes->core.bits = (psa_key_bits_t) bits;
                                     ^~~~~~~~~~~~~~~~~~~~~
    pk.c:608:12: note: 'bits' was declared here
         size_t bits;
                ^~~~

Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>

library/psa_crypto.c

@@ -420,6 +420,7 @@ psa_ecc_curve_t mbedtls_ecc_group_to_psa( mbedtls_ecp_group_id grpid,
             *bits = 448;
             return( PSA_ECC_CURVE_MONTGOMERY );
         default:
+            *bits = 0;
             return( 0 );
     }
 }