RISC-V: Minor cleanup for s extension support.

Looking at older versions of the patch, I confirmed that the odd comment
I referred to earlier was indeed from the removal of the sx support.  It
also explains an oddly formatted switch statement.  This patch fixes both
minor problems.

	bfd/
	* elfxx-riscv.c (riscv_get_prefix_class): Format s case like others.
	(riscv_parse_prefixed_ext): Fix s extension comment and reword to
	avoid over long line.

Change-Id: I1cb62e4a16188270f029b6376e4b1684000d6c7a