staging: comedi: ni_mio_common: always lock in ni_ai_poll()

`ni_ai_poll()` currently acquires (and later releases) the comedi
device's spin-lock iff `in_interrupt()` returns 0.  However, it is only
called during processing of a `COMEDI_POLL` ioctl so `in_interrupt()`
will always return 0 in this case.  Remove this test and acquire/release
the spin-lock unconditionally.  This eliminates a sparse warning about
different lock contexts for basic block.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>