drivers/flash: Fix missing ticker stop on timeout

There is timeout guard, in form of k_sem_get within work_in_time_slice,
used to detect if flash operation takes longer than expected maximum
and to return error code in such case. The processing of this timeout
is missing cancellation of scheduled ticker job that would attempt to
perform another operation. This causes problem as context for ticker
is created, on stack, within erase/write_in_timeslice which
would conclude when timeout occurs, folding the stack in process.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>