tg3: Make the RSS indir tbl admin configurable

This patch adds the ethtool callbacks necessary to change the rss
indirection table from userspace.  Should the number of interrupts
change (e.g.  across a close / open call, or through a reset) and
any one of the indirection table values fall out-of-range, the driver
will reset the indirection table to a default layout.

[Integrated many suggestions made by Ben Hutchings.]

Changes since v3

* Removed TG3_FLAG_SUPPORT_MSIX checks at the start of
  tg3_get_rxfh_indir() and tg3_set_rxfh_indir().

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>