net: designware: Fix get_timer value overflow

net: designware: Fix get_timer value overflow

get_timer returns a ulong value representing system time in ms.
On a 64-bit system, this ulong value is 64 bits long. However,
the driver stores it in a 32-bit unsigned integer, which overflows
after 49 days up time, causing the driver to get an incorrect time.

Replace the unsigned int variable with a ulong type to properly store
the value returned by get_timer.

Signed-off-by: Stanley Chu <yschu@nuvoton.com>
Signed-off-by: Jim Liu <JJLIU0@nuvoton.com>