ice: change work limit to a constant

The driver has supported a transmit work limit
that was configurable from ethtool for a long time, but
there are no good use cases for having it be a variable
that can be changed at run time.  In addition, this
variable was noted to be causing performance overhead
due to cache misses.

Just remove the variable and let the code use a constant
so that the functionality is maintained (a limit on the
number of transmits that will be cleaned in any one call
to the clean routines) without the cache miss.

Removes code, removes a variable, removes testing surface. Yay.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>