tg3: cleanup an error path in tg3_phy_reset_5703_4_5()

In the original code, if tg3_readphy() fails then it does an unnecessary
check to verify "err" is still zero and then returns -EBUSY.

My static checker complains about the unnecessary "if (!err)" check and
anyway it is better to propagate the -EBUSY error code from
tg3_readphy() instead of hard coding it here.  And really the original
code is confusing to look at.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>