platform/x86: dell-laptop: make some local functions static

Fixes the following sparse warnings:

drivers/platform/x86/dell-laptop.c:289:6: warning:
 symbol 'dell_set_arguments' was not declared. Should it be static?
drivers/platform/x86/dell-laptop.c:298:5: warning:
 symbol 'dell_send_request' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>