mmc: fix return value check condition

sd_read_ssr returns 0, means no error.
Fixes: 5b2e72f3

("mmc: read ssr only if MMC write support is enabled")

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Jean-Jacques Hiblot <jjhiblot@ti.com>