onenand: Switch to byte-based block access

Sector-based blk_write() should die; switch to byte-based
blk_pwrite() instead.  Likewise for blk_read().

This particular device picks its size during onenand_initfn(),
and can be at most 0x80000000 bytes; therefore, shifting an
'int sec' request to get back to a byte offset should never
overflow 32 bits.  But adding assertions to document that point
should not hurt.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
