mfd: rtsx: Use macros to replace some variables

mfd: rtsx: Use macros to replace some variables

In function rtsx_pci_switch_clock, some variables, such as min_n, max_n,
and max_div, are not necessary. And those assigned values look very
obscure for others. It's more proper to use macro definitions here to
replace these variables.

Signed-off-by: Wei WANG <wei_wang@realsil.com.cn>
Acked-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>