sf: spansion: fixing erasing when sector size >64KiB

The spansion_erase currently only works when the sector size is 64KB.
cmd[1] should contain the higher 8 bit of the 24 bit address of the
sector to be erased. Currently it is holding the sector index to be
erased which happens to be the same thing when the sector size is
64KB.

Signed-off-by: Marc-Andre Hebert <marc-andre.hebert@humanware.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>