vmxnet3: reduce calling vmxnet3_reg_write_inline

In output.c, we buffer the descriptors and call vmxnet3_reg_write_inline
once outside the loop. This change improves the performance dramatically.

When refilling the ring, there is no need to inform the device unless
explicitly specified by the device (ctrl.update_prod == 1)

Change-Id: I7031d58bff0d249e913d14236d416c91eb6ab94a
Signed-off-by: Steven <sluong@cisco.com>