smegw01: Fix wrong symbol override

board_mmc_get_env_part() is not called as the default implementation
of mmc_get_env_part() is used.

Fix this problem by directly calling mmc_get_env_part() instead.

Signed-off-by: Eduard Strehlau <eduard@lionizers.com>
Signed-off-by: Fabio Estevam <festevam@denx.de>