logging: Workaround build warning with clang

when building with clang we get the following warning:

log_core.c:358:40: error: use of logical '&&' with constant operand
[-Werror,-Wconstant-logical-operand]

This is because we are mix a constant IS_ENABLED(CONFIG_LOG_IMMEDIATE)
with non-constants.

Split out the check into its own statement to workaround the warning.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>