hw/sdhci: fix -Werror=maybe-uninitialized false-positive

../hw/sd/sdhci.c:846:16: error: ‘res’ may be used uninitialized [-Werror=maybe-uninitialized]

False-positive, because "length" is non-null.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

hw/sd/sdhci.c

@@ -747,7 +747,7 @@ static void sdhci_do_adma(SDHCIState *s)
     const uint16_t block_size = s->blksize & BLOCK_SIZE_MASK;
     const MemTxAttrs attrs = { .memory = true };
     ADMADescr dscr = {};
-    MemTxResult res;
+    MemTxResult res = MEMTX_ERROR;
     int i;
 
     if (s->trnmod & SDHC_TRNS_BLK_CNT_EN && !s->blkcnt) {