cm43xx: Fix PPC machine checks and match loopback gain specs

The specifications for loopback_gain calculation and for G PHY
initialization have been updated. This patch implements them and
fixes a machine check error that occurs for PPC architecture with a
phy->rev of 1.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
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