mtd: add support for switching old SST chips into QRY mode

SST 39VF160x/39VF320x and some old SST chips need a special command
sequence to enter CFI QueRY mode [1].
This patch adds the relevant sequence to cfi_qry_mode_on().
Tested with 39VF3201.

Signed-off-by: Guillaume LECERF <glecerf@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>