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 c21599a3 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 c21599a36165dbc78b380846b254017a548b9de5 "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