p_{tx,rx}_fw_statistics_pram are special: they're available only when
a device is open. If the device is closed, we should just fill the data
with zeroes.

Fixes the following oops:

root@b1:~# ifconfig eth1 down
root@b1:~# ethtool -S eth1
Unable to handle kernel paging request for data at address 0x00000000
Faulting instruction address: 0xc01e1dcc
Oops: Kernel access of bad area, sig: 11 [#1]
[...]
NIP [c01e1dcc] uec_get_ethtool_stats+0x98/0x124
LR [c0287cc8] ethtool_get_stats+0xfc/0x23c
Call Trace:
[cfaadde0] [c0287ca8] ethtool_get_stats+0xdc/0x23c (unreliable)
[cfaade20] [c0288340] dev_ethtool+0x2fc/0x588
[cfaade50] [c0285648] dev_ioctl+0x290/0x33c
[cfaadea0] [c0272238] sock_ioctl+0x80/0x2ec
[cfaadec0] [c00b5ae4] vfs_ioctl+0x40/0xc0
[cfaadee0] [c00b5fa8] do_vfs_ioctl+0x78/0x20c
[cfaadf10] [c00b617c] sys_ioctl+0x40/0x74
[cfaadf40] [c00142d8] ret_from_syscall+0x0/0x38
[...]
---[ end trace b941007b2dfb9759 ]---
Segmentation fault

p.s. While at it, also remove u64 casts, they aren't needed.
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>