rtc: fix snprintf() checking in is_rtc_hctosys()

The scnprintf() function silently truncates the printf() and returns
the number bytes that it was able to copy (not counting the NUL
terminator). Thus, the highest value it can return here is
"NAME_SIZE - 1" and the overflow check is dead code. Fix this by
using the snprintf() function which returns the number of bytes that
would have been copied if there was enough space and changing the
condition from "> NAME_SIZE" to ">= NAME_SIZE".

Fixes: 92589c986b33 ("rtc-proc: permit the /proc/driver/rtc device to use other devices")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/YJov/pcGmhLi2pEl@mwanda

rtc: core: correct trivial checkpatch warnings

Correct trivial checkpatch warnings, mostly whitespace issues and
unbalanced braces.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

rtc: convert core to SPDX identifier

Use SPDX-License-Identifier instead of a verbose license text. Also fix the
block comment alignment.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

rtc: rename core files

Rename core files so there is a clearer separation between the RTC core and
the RTC drivers.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>