isdn: eicon: fix a missing-check bug

In divasmain.c, the function divas_write() firstly invokes the function
diva_xdi_open_adapter() to open the adapter that matches with the adapter
number provided by the user, and then invokes the function diva_xdi_write()
to perform the write operation using the matched adapter. The two functions
diva_xdi_open_adapter() and diva_xdi_write() are located in diva.c.

In diva_xdi_open_adapter(), the user command is copied to the object 'msg'
from the userspace pointer 'src' through the function pointer 'cp_fn',
which eventually calls copy_from_user() to do the copy. Then, the adapter
number 'msg.adapter' is used to find out a matched adapter from the
'adapter_queue'. A matched adapter will be returned if it is found.
Otherwise, NULL is returned to indicate the failure of the verification on
the adapter number.

As mentioned above, if a matched adapter is returned, the function
diva_xdi_write() is invoked to perform the write operation. In this
function, the user command is...