mmc: card: fix bug in mmc block test

mmc unit test is run by writing to a debug-fs entry, in
which the mmc_block_test_data structure is kept as private
pointer in the file object.
This private pointer was incorrectly casted to seq_file and
the mmc_block_test_data was extracted from its private data
resulting in a bad pointer.
Fixed the code to properly extract the mmc_block_test_data
structure.

Change-Id: I8a9fea6b45b621b3b7c0abc7aaa078d340dc1c6b
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>