USB: serial: sierra driver read path bug fix

This patch fixes a problem in function sierra_indat_callback() which
would stop receiving traffic from a modem if a number of URB failures
occur.  Failed URBs are not resubmitted for the next read and there is
only a limited number of URBs allocated for the IN path. After this
number of failures, the receive path stops working on a particular
interface.

Signed-off-by: Elina Pasheva <epasheva@sierrawireless.com>