ssb: sdio: Don't overwrite const buffer if block_write fails

It doesn't make sense to clobber the const driver-side buffer, if a
write-to-device attempt failed. All other SSB variants (PCI, PCMCIA and SoC)
also don't corrupt the buffer on any failure in block_write.
Therefore, remove this memset from the SDIO variant.

Signed-off-by: Michael Büsch <m@bues.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210515210252.318be2ba@wiggum