ice: Don't reject odd values of usecs set by user

Currently if a user sets an odd [tx|rx]-usecs value through ethtool,
the request is denied because the hardware is set to have an ITR
granularity of 2us. This caused poor customer experience. Fix this by
aligning to a register allowed value, which results in rounding down.
Also, print a once per ring container type message to be clear about
our intentions.

Also, change the ITR_TO_REG define to be the bitwise and of the ITR
setting and the ICE_ITR_MASK. This makes the purpose of ITR_TO_REG more
obvious.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>