ixgbe: Clean up a useless switch statement and dead code in configure_srrctl

This patch replaces a switch statement for an 82598 workaround with an if
statement that only applies to 82598. In addition I am pulling out several
dead pieces of code and instead of reading the SRRCTL register and then
modifying it we are just writing a value which we generate from scratch.
Finally I am also removing any drop enable related code since that was
moved to a function of its own.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>