Remove the temporary @mask_, this may cause build warning when use clang
compiler for powerpc, but can't reproduce it when compile for arm64.
the build warning is caused by:

"warning: result of comparison of constant 18446744073709551615 with
expression of type (aka 'unsigned long') is always false
[-Wtautological-constant-out-of-range-compare]"

More information provided in below lore link.

After removing @mask_, there is a "CHECK:MACRO_ARG_REUSE" when run
checkpatch.pl, but due to @mask is constant, no reuse problem will happen.

Link: https://lore.kernel.org/lkml/202212160357.jJuesD8n-lkp@intel.com/t/
Fixes: 84513ecc ("phy: mediatek: fix build warning of FIELD_PREP()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230118084343.26913-1-chunfeng.yun@mediatek.comSigned-off-by: Vinod Koul <vkoul@kernel.org>