sky2: Remove redundant assignment and parentheses

The variable err will be reassigned on subsequent branches, and this
assignment does not perform related value operations. This will cause
the double parentheses to be redundant, so the inner parentheses should
be deleted.

clang_analyzer complains as follows:

drivers/net/ethernet/marvell/sky2.c:4988: warning:

Although the value stored to 'err' is used in the enclosing expression,
the value is never actually read from 'err'.

Changes in v2:

modify title category:octeontx2-af to sky2.
delete the inner parentheses.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>