board: tbs2910: Fix eMMC BOOTCFG value

Fix the BOOTCFG value for eMMC in the same way as commit
  214c3f0f
  [imx: MX6DQ{P}/DL:SABRESD Fix bmode eMMC failure]
did for sabresd.

Signed-off-by: Soeren Moch <smoch@web.de>