debug: gdbstub: ignore GDB packet exceeding buffer

If an incoming GDB packet is bigger than what the buffer can hold,
stop putting the extra characters into the buffer. This will still
read till the end to acknowledge the packet but will return error
instead. This allows the GDB session to continue instead of hanging
or timed out due to packets not being acknowledged.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>