kern_ntptime: Fix undefined behavior of the shift operator

L_LINT macro is used with negative numbers [i.e.
L_LINT(time_freq, -MAXFREQ)], it could cause undefined
behavior. It should be similar to the L_RSHIFT(v, n) macro.

MFC after: 2 weeks
Reviewed by: cy
Pull Request: https://github.com/freebsd/freebsd-src/pull/769
Signed-off-by: Dmitriy Alexandrov <d06alexandrov@gmail.com>