gdbserver/proc-service.c: bogus return value conversion

gdbserver/proc-service.c: bogus return value conversion

Building in C++ mode shows:

   int write_inferior_memory (CORE_ADDR memaddr, const unsigned char *myaddr,
       ^
  src/gdb/gdbserver/proc-service.c:93:64: error: invalid conversion from ‘int’ to ‘ps_err_e’ [-fpermissive]
     return write_inferior_memory ((unsigned long) addr, buf, size);
								  ^

It only works today by accident, write_inferior_memory does not return
a ps_err_e.

gdb/gdbserver/ChangeLog:
2015-08-27  Pedro Alves  <palves@redhat.com>

	* proc-service.c (ps_pdwrite): Return PS_ERR/PS_OK explicily.