mmc: au1xmmc: force non-modular build and remove symbol_get usage

au1xmmc is split somewhat awkwardly into the main mmc subsystem driver,
and callbacks in platform_data that sit under arch/mips/ and are
always built in.  The latter than call mmc_detect_change through
symbol_get.  Remove the use of symbol_get by requiring the driver
to be built in.  In the future the interrupt handlers for card
insert/eject detection should probably be moved into the main driver,
and which point it can be built modular again.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Manuel Lauss <manuel.lauss@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
[mcgrof: squashed in depends on MMC=y suggested by Arnd]
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>