fix(mmc): initialises response buffer with zeros

if ops->send_cmd in mmc_send_cmd returns a non-zero value, r_data
(resp_data in mmc_device_state) is never populated, so the while
condition in mmc_device_state would end up reading the uninitialized
resp_data buffer

Signed-off-by: Anand Saminathan <anans@google.com>
Change-Id: I72d752867c537d49e111e6d149c3cca122f7dc9f