am7xxx: cache device info in am7xxx_get_device_info()

Request the actual device info from the device only when there is not
a cached version of it already.

This serves two purposes:

  1. Cover the case when some firmwares provide useful device info
     only under some conditions and spit out bogus data instead when
     these conditions are not met.

  2. Speed up am7xxx_get_device_info() as the USB communication is
     performed only once. AFAWN the device info does not change
     throughout the life of an am7xxx_device.