mmc: core: Mark fixups as __maybe_unused

mmc: core: Mark fixups as __maybe_unused

Not all source files which include quirks.h make use of the all of
the available fixup information.  When this happens the compiler
complains that some constant variables are defined by never used.
We can fix this by telling the compiler that this intentional by
simply marking them as __maybe_unused.

Fixes the following W=1 kernel build warnings:

 In file included from drivers/mmc/core/sdio.c:22:
 drivers/mmc/core/quirks.h:105:31: warning: ‘mmc_ext_csd_fixups’ defined but not used [-Wunused-const-variable=]
 105 | static const struct mmc_fixup mmc_ext_csd_fixups[] = {
 | ^~~~~~~~~~~~~~~~~~
 drivers/mmc/core/quirks.h:17:31: warning: ‘mmc_blk_fixups’ defined but not used [-Wunused-const-variable=]
 17 | static const struct mmc_fixup mmc_blk_fixups[] = {
 | ^~~~~~~~~~~~~~
 In file included from drivers/mmc/core/mmc.c:25:
 drivers/mmc/core/quirks.h:123:31: warning: ‘sdio_fixup_methods’ defined but not used [-Wunused-const-variable=]
 123 | static const struct mmc_fixup sdio_fixup_methods[] = {
 | ^~~~~~~~~~~~~~~~~~
 drivers/mmc/core/quirks.h:17:31: warning: ‘mmc_blk_fixups’ defined but not used [-Wunused-const-variable=]
 17 | static const struct mmc_fixup mmc_blk_fixups[] = {
 | ^~~~~~~~~~~~~~

Cc: Andrei Warkentin <andreiw@motorola.com>
Cc: Martin Mares <mj@ucw.cz>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200701124702.908713-2-lee.jones@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>