usb: gadget: u_ether: set_wake_up_idle in rx path

The PF_WAKE_UP_IDLE flag tells the scheduler to wake up tasks that
have this flag set, or tasks woken by tasks with this flag set, on
an idle CPU if one is available. Due to high interrupts during peak
throughput testing over hsic, some task (like iperf) might get
starved if scheduled on busy cpu impacting throughput. This flag
will help tasks woken up by rndis to be scheduled on different idle
cpu and helps with throughput.

Change-Id: I10db8209bf5396ae8f86f682df374c006bbcc7f0
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>