mfd: intel-m10-bmc: Move m10bmc_sys_read() away from header

Move m10bmc_sys_read() out from the header to prepare it for adding
more code into the function which would make it too large to be a
static inline any more.

While at it, replace the vague wording in function comment with more
precise statements.

Reviewed-by: Russ Weight <russell.h.weight@intel.com>
Acked-by: Guenter Roeck <linux@roeck-us.net> # For hwmon
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230417092653.16487-4-ilpo.jarvinen@linux.intel.com