usb: gadget: s3c-hsotg: fix interrupt configuration in dedicated-fifo mode

In dedicated-fifo mode TxFIFOEmpty interrupt should be asserted when TxFIFO
for this endpoint is completly empty, so NPTxFEmpLvl and PTxFEmpLvl bits are
set in GAHBCFG register.
In DIEPMSK register INTknTXFEmpMsk is set, becouse it's needed to indicate
FIFO Empty state.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>