Redefine ADJFREQ_MIN to avoid undefined behaviour (when not using -fwrapv)

Redefine ADJFREQ_MIN to avoid undefined behaviour (when not using -fwrapv)

Change the definition of ADJFREQ_MIN so that it does not shift
a negative value. Such shifting is undefined in standard C.

This came up when cross-compiling the kernel using ports clang.
The shifting becomes defined when compiling with option -fwrapv.
Base clang enables this option by default.

OK naddy@ cheloha@