e1000: FIX: fix wrong txdctl threshold bitmasks

Threshold bitmasks for prefetch, host and writeback were clearing
bits that they were not supposed to. The leftmost 2 bits in the byte
for each threshold are reserved.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>