iceprog: Better reset of flash

If the flash was in a 'weird' mode (like CRM, QPI, ...), sending
a continuous stream of 0xff should take it out of it.

It looks like the previous code was trying to do that, but
was only transferring 8 bits and then 2 bits which is way
too short.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>