platform/x86: thinkpad_acpi: Replace next_cmd(&buf) with strsep(&buf, ",")

It seems next_cmd() predates the strsep() implementation in the kernel.
For a long time we have the latter one, thus, replace next_cmd(&buf) with
strsep(&buf, ",").

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>