mmc: core: don't initialize block size from ext_csd if not present

If extended CSD was not available, the eMMC driver would incorrectly
set the block size to 0, as the data_sector_size field of ext_csd
was never initialized. This issue was exposed by commit 817046ecddbc
("block: Align max_hw_sectors to logical blocksize") which caused
max_sectors and max_hw_sectors to be set to 0 after setting the block
size to 0, resulting in a kernel panic in bio_split when attempting
to read from the device. Fix it by only reading the block size from
ext_csd if it is available.

Fixes: a5075eb94837 ("mmc: block: Allow disabling 512B sector size emulation")
Signed-off-by: Peter Collingbourne <[email protected]>
Reviewed-by: Damien Le Moal <[email protected]>
Link: https://linux-review.googlesource.com/id/If244d178da4d86b52034459438fec295b02d6e60
Acked-by: Adrian Hunter <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Ulf Hansson <[email protected]>

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