mmc: dw_mmc: Remove set-but-unused variable.

count is only ever used by assigning to old_len if count == 0, and
then old_len isn't ever used at all.  So, both are redundant.  Fixes:

drivers/mmc/host/dw_mmc.c: In function ‘dw_mci_read_data_pio’:
drivers/mmc/host/dw_mmc.c:1034:32: warning: variable ‘old_len’ set but
not used [-Wunused-but-set-variable]

Signed-off-by: Chris Ball <cjb@laptop.org>
Acked-by: Will Newton <will.newton@imgtec.com>