usb: musb: gadget: clear TXPKTRDY flag when set FLUSHFIFO

Fixes mis-use of MUSB's hardware feature where it won't
flush FIFOs when TXPKTRDY flag was set before and we are
flushing setting both FLUSHFIFO and TXPKTRDY.

In other words, we need to ensure that when we try to
flush FIFOs, we don't accidentaly set TXPKTRDY bit too
due to a read-back of the register.

The MUSB Programming Guide says "May be set simultaneously
with TxPktRdy to abort the packet that is currently being
loaded into the FIFO". This is a situation where TXPKTRDY
hasn't been set yet, but some data already loaded into the
fifo. It looks, that if TXPKTRDY has been set before, and
there is no loading in progress, but we set FLUSHFIFO with
the TXPKTRDY, controller tries to use the same logic to
abort loading and as the result just does nothing (because
there is no packet been loaded currently)

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@nokia.com>

[ balbi@ti.com : fixed one whitespace git complained about
		 improved the commit log slightly ]

Signed-off-by: Felipe Balbi <balbi@ti.com>