tg3: Change nvram command timeout value to 50ms

Commit 506724c4 "tg3: Override clock,
link aware and link idle mode during NVRAM dump" changed the timeout
value for nvram command execution from 100ms to 1ms. But the 1ms
timeout value was only sufficient for nvram read operations but not
write operations for most of the devices supported by tg3 driver.
This patch sets the MAX to 50ms. Also it uses usleep_range instead
of udelay.

Signed-off-by: Prashant Sreedharan <prashant@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Suggested-by: David Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>