gdbserver: Rephrase loops in regsets_fetch/store_inferior_registers

Replace the while-loops in linux-low.c that iterate over regsets by
for-loops.  This makes it clearer what is iterated over.  Also, since
"continue" now moves on to the next iteration without having to
increment the regset pointer first, the code is slightly reduced.

In case of EIO the old code did not increment the regset pointer, but
iterated over the same (now disabled) regset again.  This extra
iteration is now avoided.

gdb/gdbserver/ChangeLog:

	* linux-low.c (regsets_fetch_inferior_registers): Rephrase
	while-loop as for-loop.
	(regsets_store_inferior_registers): Likewise.