Input: ad7877 - switch to using threaded IRQ

Instead of using asynchronous SPI API and then spinning waiting for SPI
transfer to complete when disabling the device, let's use threaded IRQ
model and spi_sync().

Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Tested-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>