fix(st): reduce MMC block_buffer

This buffer used by io_block should be the size on one block (512 bytes)
not 512*4 due to the use of uint32_t. Change that to uint8_t. This
reduces the size of BSS by 1.5KiB.

Change-Id: I8492f13f5386679b0c81efba66911422412782f9
Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
1 file changed