USB: musb: be careful with 64K+ transfer lengths (gadget side)

request->actual is an unsigned and we should use the same
variable type for fifo_count otherwise we might lose some
data if request->length >= 64kbytes.

[ dbrownell@users.sourceforge.net: fix compiler warning ]

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>