fix(ufs): don't zero out the write buffer

Previously ufs_write_blocks was memsetting the write buffer before
calling ufs_prepare_cmd, causing zeros to be written to UFS. This change
deletes the memset call so the original buffer contents get written to
UFS.

Signed-off-by: Jorge Troncoso <jatron@google.com>
Change-Id: I3299f11b30e6d7d409408ce11a6759c88607ee18