ata: sata_mv: remove a redundant assignment to pointer ehi

The pointer ehi is being assigned to a value that is never read
and is redundant.  Clean up the code and move the ehi declaration
and initialization to the code block where it is used. Cleans up
clang warning: Value stored to 'ehi' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Tejun Heo <tj@kernel.org>