mmc: sd: Fix SDR12 timing regression

This patch fixes a failure to recognize SD cards reported on a Dell
Vostro with O2 Micro SD card reader.  Patch 49c468fc ("mmc: sd: add
support for uhs bus speed mode selection") caused the problem, by
setting the SDHCI_CTRL_HISPD flag even for legacy timings.

Signed-off-by: Alexander Elbs <alex@segv.de>
Acked-by: Philip Rakity <prakity@marvell.com>
Acked-by: Arindam Nath <arindam.nath@amd.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>