tty: smux_ctl: Do not set POLLERR if signal received

During the userspace freeze process, a fake signal is sent to userspace
applications.  In the case of SMUX CTL, the smux_ctl_poll() will set the
POLLERR flag if a signal is received.  This incorrectly unblocks the
userspace application that called poll() and the userspace application
will grab a wakelock before doing additional processing which aborts the
freeze process.

Change smux_ctl_poll() to not set POLLERR if a signal is received.

CRs-Fixed: 402651
Change-Id: I3d18552ceff769d302d1536cf5b9345f3aad767b
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>