tty: n_smux: Mask power collapse support

If power-collapse is supported locally, then SMUX will always set the
power-collapse supported flag in an OPEN ACK which may cause confusion
for the remote side.

Mask power-collapse enabled with the value in the remote open
acknowledge message.

CRs-Fixed: 443770
Change-Id: I8ceb753e25a53d13fa5b5288457e8b1993452a18
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>