Staging: sxg: fix build warnings in sxg_ethtool

This fixes some build problems with the ethtool support recently added
for the sxg driver.

This is a very good example of why you need to check the build for
warnings and then fix them...

Cc: Mithlesh Thukral <mithlesh@linsyssoft.com>
Cc: LinSysSoft Sahara Team <saharaproj@linsyssoft.com>
Cc: Christopher Harrer <charrer@alacritech.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>