sdio: Adjust the location of the GXL best sampling point

sdio: Adjust the location of the GXL best sampling point

PD#160789: sdio: optimizing the selection of sdio tuning sampling points

[problem] When all sampling points can be sampled correctly,

select 0 points as the best sampling point.M8 platform is normal,

but some S905 platform is not suitable.

[resolved] The GXL platfoarm selects the intermediate

point of all sampling points as the best sampling point

Change-Id: I59097daa8fe3d99f2f11a3e1347945c8eea4e1c9
Signed-off-by: Long Yu <long.yu@amlogic.com>