fsl_*_serdes.c: Modify memset call in serdes_init

GCC 5.x does not like sizeof(array_variable) and errors out.  Change these
calls to be instead sizeof(u8) (as that's what serdes_prtcl_map is) *
SERDES_PRCTL_COUNT (the number of array elements).

Cc: York Sun <yorksun@freescale.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: York Sun <yorksun@freescale.com>