hwclock: --epoch presence test fails

hwclock: --epoch presence test fails

hwclock --setepoch --epoch 0
Will warn that the epoch option is required.

The --epoch presence test is made on its argument after it is
converted to an integer. This means any value it can be tested
for, can also be given as an input.

So make the conversion after the presence test, like the
--date option does.

Signed-off-by: J William Piggott <elseifthen@gmx.com>