mmc: Add missing sg_init_table() call

mmc_init_queue only initializes the scatterlists with sg_init_table()
when using a bounce buffer. This leads to a BUG() when CONFIG_DEBUG_SG
is set.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>