Increase ChibiOS's PORT_INT_REQUIRED_STACK stack size to 48

It seems that we can overflow the stack in the idle thread if the code size
hits a certain limit. For debugging the code is usually bigger anyway,
so it's a good idea to reserve a bit more.

One can always drop it for "production code", if need be.

Signed-off-by: Kalle Vahlman <kalle.vahlman@snowcap.fi>