isdn: hisax: set error code on failure

In function hfc4s8s_probe(), the value of return variable err should be
negative on failures. However, when the call to request_region() returns
NULL, the value of err is 0. This patch fixes the bug, assigning
"-EBUSY" to err on the path that request_region() fails.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=188931

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>