Input: nspire-keypad - enable interrupts only when opened

The driver registers an interrupt handler in _probe, but didn't configure
them until later when the _open function is called. In between, the keypad
can fire an IRQ due to touchpad activity, which the handler ignores. This
causes the kernel to disable the interrupt, blocking the keypad from
working.

Fix this by disabling interrupts before registering the handler.
Additionally, disable them in _close, so that they're only enabled while
open.

Fixes: fc4f3146

 ("Input: add TI-Nspire keypad support")
Signed-off-by: Fabian Vogt <fabian@ritter-vogt.de>
Link: https://lore.kernel.org/r/3383725.iizBOSrK1V@linux-e202.suse.de
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>