echoplayer: re-enable 50 MHZ SDMMC bus

The problem with drive strength and things not working reliably
at 50 MHz turned out to be entirely caused by the ESD diodes on
the data/command/clock lines, which have a whopping 200-300 pF
junction capacitance -- 6-10x higher than the 30 pF limit given
by the MMC spec.

After desoldering the diodes the bus seems stable at 50 MHz and
with any drive strength (note MEDIUM is still fast enough even
for 50 MHz, so the drive strength is unchanged).

Change-Id: If9847ee4145f5ed2f7e172cfa89acad0737a897f

firmware/target/arm/stm32/echoplayer/sdmmc-echoplayer.c

@@ -46,7 +46,8 @@ static const struct sdmmc_host_config sdmmc_config INITDATA_ATTR = {
     .bus_widths = SDMMC_BUS_WIDTH_1BIT |
                   SDMMC_BUS_WIDTH_4BIT,
     .bus_clocks = SDMMC_BUS_CLOCK_400KHZ |
-                  SDMMC_BUS_CLOCK_25MHZ,
+                  SDMMC_BUS_CLOCK_25MHZ |
+                  SDMMC_BUS_CLOCK_50MHZ,
     .max_nr_blocks = 65535,
     .is_removable = true,
 };