mbedtls: Fix reads getting stuck when the socket has disconnected

We've seen this behaviour when iOS resumes from sleep:

dbg> 0x11cd03750: ssl err dbg> lws_ssl_capable_read: WANT_READ
dbg> 0x11cd03750: LWS_SSL_CAPABLE_MORE_SERVICE
dbg> SSL Capable more service
dbg> 0x11cd03750: SSL_read says -1
dbg> 0x11cd03750: ssl err 2 errno 57
dbg> lws_ssl_capable_read: WANT_READ
dbg> 0x11cd0375dbg> SSL Capable more service
dbg> 0x11cd03750: SSL_read says -1
dbg> 0x11cd03750: ssl err 2 errno 57
dbg> lws_ssl_capable_read: WANT_READ