bas_gigaset: correctly allocate USB interrupt transfer buffer

Every USB transfer buffer has to be allocated individually by kmalloc.

Impact: bugfix, no functional change

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Tested-by: Kolja Waschk <kawk@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>