mmc: sdhci-cadence: Fix -Wuninitialized in sdhci_cdns_tune_blkgap()

mmc: sdhci-cadence: Fix -Wuninitialized in sdhci_cdns_tune_blkgap()

Clang warns (or errors with CONFIG_WERROR=y):

  drivers/mmc/host/sdhci-cadence.c:297:9: error: variable 'hrs37_mode' is uninitialized when used here [-Werror,-Wuninitialized]
    297 |         writel(hrs37_mode, hrs37_reg);
        |                ^~~~~~~~~~
  drivers/mmc/host/sdhci-cadence.c:291:16: note: initialize the variable 'hrs37_mode' to silence this warning
    291 |         u32 hrs37_mode;
        |                       ^
        |                        = 0

A previous revision assigned SDHCI_CDNS_HRS37_MODE_MMC_HS200 to
hrs37_mode in a switch statement but the final revision moved to a
simple if statement. Pass that as the value to writel() and
remove hrs37_mode, clearing up the warning.

Fixes: 60613a8b9b81 ("mmc: sdhci-cadence: implement multi-block read gap tuning")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Benoît Monin <benoit.monin@bootlin.com>
Link: https://lore.kernel.org/r/20250819-mmc-sdhci-cadence-fix-uninit-hrs37_mode-v1-1-94aa2d0c438a@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>