Fixup: Use TC ECC type instead of uint32_t during pubkey parsing

Using explicit enumerated types avoids type confusion.
diff --git a/library/pkparse.c b/library/pkparse.c
index a3184bf..d03695b 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -182,7 +182,7 @@
 #if defined(MBEDTLS_USE_TINYCRYPT)
 static int pk_use_ecparams( const mbedtls_asn1_buf *params )
 {
-    uint32_t grp_id;
+    mbedtls_uecc_group_id grp_id;
 
     if( params->tag == MBEDTLS_ASN1_OID )
     {