net: stmmac: ptp: fix potentially overflowing expression

Convert the u32 variable to type u64 in a context where expression of
type u64 is required to avoid potential overflow.

Fixes: e9e37200 ("net: stmmac: ptp: update tas basetime after ptp adjust")
Signed-off-by: Xiaoliang Yang <xiaoliang.yang_1@nxp.com>
Link: https://lore.kernel.org/r/20211223073928.37371-1-xiaoliang.yang_1@nxp.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>