gigaset: fix possible oops in error handling

Use pr_warning() / pr_err() instead of dev_warn() / dev_err() in two
places where the dev pointer isn't guaranteed to be valid.

Impact: error handling bugfix
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>