hw/nand.c: correct the sense of the BUSY/READY status bit

The BIT6 of Status Register(SR):

SR[6] behaves the same as R/B# pin
    SR[6] = 0 indicates the device is busy;
    SR[6] = 1 means the device is ready

Some NAND flash controller (i.e. ftnandc021) relies on the SR[6]
to determine if the NAND flash erase/program is success or error timeout.

P.S:
The exmaple NAND flash datasheet could be found at following link:
http://www.mxic.com.tw/QuickPlace/hq/PageLibrary4825740B00298A3B.nsf/h_Index/8FEA549237D2F7674825795800104C26/$File/MX30LF1G08AA,%203V,%201Gb,%20v1.1.pdf

Signed-off-by: Kuo-Jung Su <dantesu@gmail.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>