[TG3]: Refine nvram locking

Add nvram lock count so that calls to tg3_nvram_lock()/unlock() can
be nested. Add error checking to all callers of tg3_nvram_lock()
where appropriate. To prevent nvram lock failures after halting the
firmware, it is also necessary to release firmware's nvram lock in
tg3_halt_cpu().

Update version to 3.48.

Based on David Miller's initial patch.

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