serial: Remove extraneous SYS_MALLOC_F check

We enforce that DM_SERIAL will have SYS_MALLOC_F enabled and so
SYS_MALLOC_F_LEN will have a value.  Remove the build-time check.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>