gpio: mockup: improve the debugfs input sanitization
We're currently only checking the first character of the input to the
debugfs event files, so a string like '0sdfdsf' is valid and indicates
a falling edge event.
Be more strict and only allow '0', '1', '0\n' & '1\n'.
While we're at it: move the sanitization code before the irq_enabled
check so that we indicate an error on invalid input even if nobody is
waiting for events.