USB: whci-hcd: Fix potential memory leak in qset_add_urb_sg()

Do not leak memory by updating pointer with potentially
NULL realloc return value.

By the way remove unused local variable:
	struct whc_page_list_entry *entry;
More precisely, it was used to increment uninitialized value within one of cycles.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>