Input: ads7846 - fix unsafe disable_irq

The use of disable_irq inside the handler for the interrupt being
disabled has always been dangerous.  disable_irq should wait for that
handler to complete before returning -> deadlock.

For some reason this wasn't actually the case until 3aa551c9

 was merged
but since this time, the ads7846 driver has deadlocked the system on
first interrupt.

Convert the driver to use the handler-safe _nosync variant.

Signed-off-by: Ben Nizette <bn@niasdigital.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>