modules: hal_ethos_u: fix log configuration in CMake file

The core driver ETHOSU_LOG_SEVERITY CMake variable is a cache variable,
and thus will not be configured by the ETHOSU_LOG_SEVERITY CMake
variable of the hal_ethos_u module with local scope.
Change ETHOSU_LOG_SEVERITY local variable to be a cache var.

In addition, change condition test from LESS_EQUAL to LESS in order
to prevent an out-of-range index when accessing list variable.

Signed-off-by: Ledion Daja <ledion.daja@arm.com>