gigaset: use dev_ macros for messages

The info() / warn() / err() macros from usb.h for generating kernel
messages are considered inferior to dev_info() / dev_warn() / dev_err()
from device.h.  Replace them where possible.  Also correct the severity
level and improve the text of one message.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>