tools: mtd-utils: fix disabling tests

tools: mtd-utils: fix disabling tests

Passing --disable-tests does not do anything since upstream commit [0]
("Fix test binary installation") as that commit removed it since there is
already and existing --without-tests compile option to not compile
the tests at all as --disable-tests was just disabling their installation.

So, lets just pass --without-tests instead to disable test compilation.

[0] https://git.infraroot.at/mtd-utils.git/commit/?id=7170a28d46d5db1e7a9da24a5555a194a233ef0b

Fixes: 67efb6a66146 ("tools: mtd-utils: Update to version 2.1.4")
Link: https://github.com/openwrt/openwrt/pull/15791
Signed-off-by: Robert Marko <robimarko@gmail.com>