char: nwbutton: open-code interruptible_sleep_on

The nwbutton driver uses interruptible_sleep_on to wait for buttons
getting pressed after we enter the read() function, which is inherently
racy and cannot be fixed by using wait_event without changing the
driver's user space interface.

Instead, this patch just uses an open-coded variant of the same
interruptible_sleep_on() call, so the driver behavior doesn't change
but we remove the sleep_on family from the kernel.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>