mmc: dw_mmc: fixed wrong bit operation for SDMMC_GET_FCNT()

In status register, fifo_count is bit[29:17].
(0x1FFF is correct)

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>