This patch removes an unnecessary check of unsigned expression >= 0,
which also fixes the following clang warnings:
sierra/mm-broadband-modem-sierra.c:570:18: error: comparison of
unsigned expression >= 0 is always true
[-Werror,-Wtautological-compare]
mode >= 0 &&
~~~~ ^ ~
Bug reported on https://code.google.com/p/chromium/issues/detail?id=235989
Patched by Yunlian Jiang <yunlian@chromium.org>