memstick: r592: ignore kfifo_out() return code again

A minor cleanup to address a clang warning removed an assigned
but unused local variable, but this now caused a gcc warning as
kfifo_out() is annotated to require checking its return code:

In file included from drivers/memstick/host/r592.h:13,
                 from drivers/memstick/host/r592.c:21:
drivers/memstick/host/r592.c: In function 'r592_flush_fifo_write':
include/linux/kfifo.h:588:1: error: ignoring return value of '__kfifo_uint_must_check_helper' declared with attribute 'warn_unused_result' [-Werror=unused-result]
  588 | __kfifo_uint_must_check_helper( \
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  589 | ({ \
      | ~~~~
  590 |         typeof((fifo) + 1) __tmp = (fifo); \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  591 |         typeof(__tmp->ptr) __buf = (buf); \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  592 |         unsigned long __n = (n); \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~
  593 |         const size_t __recsize = sizeof(*__tmp->rectype); \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  594 |         struct __kfifo *__kfifo = &__tmp->kfifo; \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  595 |         (__recsize) ?\
      |         ~~~~~~~~~~~~~~
  596 |         __kfifo_out_r(__kfifo, __buf, __n, __recsize) : \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  597 |         __kfifo_out(__kfifo, __buf, __n); \
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  598 | }) \
      | ~~~~
  599 | )
      | ~
drivers/memstick/host/r592.c:367:9: note: in expansion of macro 'kfifo_out'
  367 |         kfifo_out(&dev->pio_fifo, buffer, 4);
      |         ^~~~~~~~~

The value was never checked here, and the purpose of the function
is only to flush the contents, so restore the old behavior but
add a cast to void and a comment, which hopefully warns with neither
gcc nor clang now.

If anyone has an idea for how to fix it without ignoring the return
code, that is probably better.

Fixes: 4b00ed3c

 ("memstick: r592: remove unused variable")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210421135215.3414589-1-arnd@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>