[ottf, spi console] Change spi buffer last read addr contraint to open ended

[BenBenderOT]

Description

The current comparison to spi last read addr is an exact comparison to the bytes read.
This causes an incompatability for ATE tester that use multisite spi reads and the length of the reads are not the same.
The suggestion is to a minimum spi buffer read and as long as that minimum is met the fw can continue.
See my comment:
#28734 (comment)