cfi_flash: precision and underflow problem in tout calculation

With old configuration it could happen tout=0 if CONFIG_SYS_HZ<1000.

Signed-off-by: Renato Andreola <renato.andreola@imagos.it>
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Stefan Roese <sr@denx.de>