xtensa: ISS: don't hang if stdin EOF is reached

xtensa: ISS: don't hang if stdin EOF is reached

commit 362014c8d9d51d504c167c44ac280169457732be upstream.

Simulator stdin may be connected to a file, when its end is reached
kernel hangs in infinite loop inside rs_poll, because simc_poll always
signals that descriptor 0 is readable and simc_read always returns 0.
Check simc_read return value and exit loop if it's not positive. Also
don't rewind polling timer if it's zero.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>