MFC: r338304

The read accessors generated by BUS_ACCESSOR() have the problem that
they don't check the result of BUS_READ_IVAR(9) and silently return stack
garbage on failure in case a bus doesn't implement a particular instance
variable for example. With MMC bridges not providing MMCBR_IVAR_RETUNE_REQ,
yet, this in turn can cause mmc(4) to get into a state in which re-tuning
seems to be necessary but is inappropriate, causing mmc_wait_for_request()
to fail. Thus, don't use BUS_ACCESSOR() for mmcbr_get_retune_req() and
instead provide a version of the latter which returns retune_req_none if
reading MMCBR_IVAR_RETUNE_REQ fails.