mlxsw: spectrum_ptp: Fix compilation on 32-bit ARM

Compilation on 32-bit ARM fails after commit 992aa864 ("mlxsw:
spectrum_ptp: Add implementation for physical hardware clock operations")
because of 64-bit division:

arm-linux-gnueabi-ld:
drivers/net/ethernet/mellanox/mlxsw/spectrum_ptp.o: in function
`mlxsw_sp1_ptp_phc_settime': spectrum_ptp.c:(.text+0x39c): undefined
reference to `__aeabi_uldivmod'

Fix by using div_u64().

Fixes: 992aa864

 ("mlxsw: spectrum_ptp: Add implementation for physical hardware clock operations")
Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>