wlcore: don't allow SDIO read/writes after failure

Set a flag and after the first read/write failure is encountered.
This flag will disallow further SDIO read/writes until op_stop() is
executed, which will clear all flags.

This prevents further errors from occurring, since one error usually
indicates that IO operations won't work anymore until the chip is
rebooted.  By blocking more calls, we avoid extra timeouts and having
to wait for them to occur.

[Added second paragraph explaining why the change is needed. -- Luca]

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>