net: wwan: t7xx: Avoid calls to skb_data_area_size()

skb_data_area_size() helper was used to calculate the size of the
DMA mapped buffer passed to the HW. Instead of doing this, use the
size passed to allocate the skbs.

Signed-off-by: Ricardo Martinez <ricardo.martinez@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>