uboot-envtools: ipq807x: remove number of blocks and code simplification

Envtools can automatically detect the number of blocks.

Signed-off-by: Paweł Owoc <frut3k7@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/17463


Signed-off-by: Robert Marko <robimarko@gmail.com>