readclock/tda19988: fix compiler/cppcheck warnings

readclock:
 - the setup function for readclock on i386 was missing a return.
 - the message field for RTCDEV_GRANT wasn't the right type.

tda19988:
 - position variable was used without being initialized.

Change-Id: I3f78dd0a9d44f35eb38cb7da0974909f066b7007