Fix remote/17230

Fix remote/17230

This patch is to fix PR remote/17230, which is a leftover of the
to_xfer_partial interface change.  I tried splint and it reprots this
problem like this,

../../../git/gdb/remote-mips.c:2236: Return value type unsigned long long does not match declared type enum target_xfer_status: len

and this problem only exists in remote-mips.c.

gdb:

2014-08-07  Yao Qi  <yao@codesourcery.com>

	PR remote/17230
	* remote-mips.c (mips_xfer_memory): Set *xfered_len and return
	TARGET_XFER_OK instead of 0.