uinput device is always available for writing so we should always report
EPOLLOUT and EPOLLWRNORM bits, not only when there is nothing to read from
the device.

Fixes: d4b675e1 ("Input: uinput - fix returning EPOLLOUT from uinput_poll")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20191209202254.GA107567@dtor-wsSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>