modem: hl7800: Fix build issue with arm-clang

When building drivers.modem.build we get a link error since
log_source_id_get() is not defined when CONFIG_LOG_MODE_MINIMAL
is defined.

Change ifdef protection to be CONFIG_LOG_RUNTIME_FILTERING
instead of CONFIG_LOG in the code to handle this case.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>