rtc: sysfs: fix a cast removing the const attribute

The char pointer buf_ptr is assigned an address from a const char
pointer buf (parameter of wakealarm_store).
The data pointer by buf_ptr is never modified.
So casting it to a (char *) is useless.

This patch remove this cast, and transform buf_ptr to a const char pointer.

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>