Replace 0 with TARGET_XFER_EOF in to_xfer_partial

Replace 0 with TARGET_XFER_EOF in to_xfer_partial

I run splint in gdb source and get the following warnings:

  ../../../git/gdb/corelow.c:740: Return value type int does not match declared type enum target_xfer_status: 0

'TARGET_XFER_EOF' (enum target_xfer_status) is expected to be returned,
but 0 is returned.  This patch is to replace 0 with TARGET_XFER_EOF
in the implementations of to_xfer_partial.

gdb:

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

	* corelow.c (core_xfer_partial): Replace 0 with TARGET_XFER_EOF.
	* remote-m32r-sdi.c (m32r_xfer_memory): Likewise.
	* remote.c (remote_read_bytes): Likewise.