net: usb: pegasus: Check the return value of get_geristers() and friends;

Certain call sites of get_geristers() did not do proper error handling.  This
could be a problem as get_geristers() typically return the data via pointer to a
buffer.  If an error occurred the code is carelessly manipulating the wrong data.

Signed-off-by: Petko Manolov <petkan@nucleusys.com>
Reviewed-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>