sky2: Replace mdelay with msleep in sky2_vpd_wait

sky2_vpd_wait is not called in an interrupt handler nor holding a spinlock.
The function mdelay in it can be replaced with msleep, to reduce busy wait.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>