eeprom: idt_89hpesx: replace open-coded kmemdup_nul

eeprom: idt_89hpesx: replace open-coded kmemdup_nul

A malloc + strncpy + manual NUL_termination is just kmemdup_nul. Let's use
this interface as it is less error-prone and more readable.

Also drop `csraddr_len` as it is just used in a single place and we can
just do the arithmetic in-line.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230927-strncpy-drivers-misc-eeprom-idt_89hpesx-c-v1-1-08e3d45b8c05@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>