cris: move usec/nsec conversion to do_slow_gettimeoffset

Move usec to nsec conversion from arch_gettimeoffset() to
do_slow_gettimeoffset(); in a future patch, do_slow_gettimeoffset()
will be used directly as the implementation of arch_gettimeoffset(),
so needs to perform all required calculations.

Cc: Mikael Starvik <starvik@axis.com>
Cc: linux-cris-kernel@axis.com
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>