mmc: dw_mmc: miscaculated the fifo-depth with wrong bit operation

In FIFOTH register, the RX_WMark field (bits[27:16]) defaults to
FIFO_DEPTH - 1. When reading it, bits[26:16] were being used, so
fix it to use the mask 0xfff instead of 0x7ff.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>