tty: n_smux: Delete power command from list

Remove power command from list before deleting the packet.

Without this fix, if there is a pending power command during a subsystem
restart or a line-discipline unload, then the code will end up in an
infinite loop until the watchdog kicks in and an access after-free check
fails.

Change-Id: Ib8f24598ce1273411a2674d5d61634ffb1944bb1
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>