ipc: icmsg: increase stack size of RX work queue thread

Increases default size of stack used by work queue RX thread
from 1024 to 1280 to avoid memory issues.

After adding the buffer with the default size of 128
that is allocated on the thread's stack (see #77552),
there is too little stack left during heavy stress.

Signed-off-by: Piotr Koziar <piotr.koziar@nordicsemi.no>