RISC-V: Remove a loop in the ISA parser

Since commit e601909a3287bf541c6a7d82214bb387d2c76d82 ("RISC-V: Support
to parse the multi-letter prefix in the architecture string.") changed
so that all prefixed extensions are parsed in single
riscv_parse_prefixed_ext call, a "while" loop on riscv_parse_subset
is no longer required.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_parse_subset): Remove unnecessary loop.
diff --git a/bfd/elfxx-riscv.c b/bfd/elfxx-riscv.c
index d74e7a5..2915b74 100644
--- a/bfd/elfxx-riscv.c
+++ b/bfd/elfxx-riscv.c
@@ -2019,14 +2019,11 @@
   if (p == NULL)
     return false;
 
-  /* Parse the different classes of extensions in the specified order.  */
-  while (*p != '\0')
-    {
-      p = riscv_parse_prefixed_ext (rps, arch, p);
+  /* Parse prefixed extensions.  */
+  p = riscv_parse_prefixed_ext (rps, arch, p);
 
-      if (p == NULL)
-        return false;
-    }
+  if (p == NULL)
+    return false;
 
   /* Finally add implicit extensions according to the current
      extensions.  */