xhci: don't re-enable IE constantly

xhci: don't re-enable IE constantly

While we're at that, define IMAN bitfield to aid readability.

The interrupt enable bit should be set once on driver init, and we
shouldn't need to continually re-enable it.  Commit a448445d introduced
a read of the irq_pending register, and that allows us to preserve the
state of the IE bit.  Before that commit, we were blindly writing 0x3 to
the register.

This patch should be backported to kernels as old as 2.6.36, or ones
that contain the commit a448445d9a38e1ff1e71de6de570e706c24e1db1 "USB:
xhci: Reduce reads and writes of interrupter registers".

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org