usb: isp1760: set IRQ flags properly

The IRQF_DISABLED is a NOOP and scheduled to be removed.  According to
commit e58aa3d2 ("genirq: Run irq handlers with interrupts
disabled") running IRQ handlers with interrupts enabled can cause stack
overflows when the interrupt line of the issuing device is still active.

This patch removes using this deprecated flag and additionally removes
redundantly setting IRQF_SHARED for isp1760_udc_register().

Signed-off-by: Valentin Rothberg <Valentin.Rothberg@lip6.fr>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>