usb: musb: gadget: fix bulk IN infinit hangs in double buffer case

This patch fixes one infinite hang of bulk IN transfer in double buffer
case, the hang can be observed easily by test #6 of usbtest if musb is
configured as g_zero and fifo mode 3 is taken to enable double fifo.

In fact, the patch only removes the check for non-empty fifo before
loading data from new request into fifo since the check is not correct:

	-in double buffer case, fifo may accommodate more than one packet,
	even though it has contained one packet already and is non-empty

	-since last DMA is completed before calling musb_g_tx, it is sure
	that fifo may accommodate at least one packet

Without applying the patch, new requst enqueued from .complte may not
have a chance to be loaded into fifo, then will never be completed and
cause infinite hangs.

With the patch, on my beagle B5, test#6(queued bulk in) can be passed and
test result may go beyond 33Mbyte/s if musb is configured as g_zero and
fifo mode 3 is taken, follows the test command:

	#testusb -D DEV_NAME -c 1024 -t 6 -s 32768 -g 8   [1]

[1],
    -source of testusb : tools/usb/testusb.c under linux kernel;

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Acked-by: Anand Gadiyar <gadiyar@ti.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: Anand Gadiyar <gadiyar@ti.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>