The initial code used roundup() to round the starting time to
a multiple of a period.  This generated an error on 32-bit
systems, so was replaced with DIV_ROUND_UP_ULL().

However, this truncates to 32-bits on a 64-bit system.  Replace
with DIV64_U64_ROUND_UP() instead.

Fixes: b325af3c ("ptp: ocp: Add signal generators and update sysfs nodes")
Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Link: https://lore.kernel.org/r/20220506223739.1930-2-jonathan.lemon@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>