cfi-flash: Fix problem in flash_toggle(), busy was not detected reliably

This patch simplifies flash_toggle() (AMD commandset), which is used to
detect if a FLASH device is still busy with erase/program operations. On
800MHz Canyonlands/Glacier boards (460EX/GT) the current implementation
did not detect the busy state reliably, resulting in non erased sectors
etc. This patch now simplifies this function by "just" comparing the
complete data-word instead of ANDing it with the command-word (0x40)
before the compatison. It is done the same way in the Linux implementation
chip_ready() in cfi_cmdset_0002.c.

Signed-off-by: Stefan Roese <sr@denx.de>