Initialized "cgmi_retries" variable from CustomInitContext with the
same value as the other retries, moreover the context is now allocated
with g_slice_new0.
Before this changes, when cgmi_retries assumed big values during the
probing of no AT-capable ports, the command AT+CGMI (mm-plugin-dell.c:custom_init_step)
was sent a semi-infinite number of times, blocking the plugin's initialization.
8a38621869