usb: dwc2: gadget: write correct value in ahbcfg register

HBstLen is GAHBCFG[4:1]. Use GAHBCFG_HBSTLEN_SHIFT to write burst-
length at correct position.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>