wifi: ipw2x00: refactor to use kstrtoul

The current implementation seems to reinvent what `kstrtoul` already does
in terms of functionality and error handling. Remove uses of `simple_strtoul()`
in favor of `kstrtoul()`.

There is the following note at `lib/vsprintf.c:simple_strtoull()` which
further backs this change:
| * This function has caveats. Please use kstrtoull (or kstrtoul) instead.

And here, simple_str* are explicitly deprecated [3].

This patch also removes an instance of the deprecated `strncpy` which helps [2].

Compile tested only.

Link: https://lore.kernel.org/all/202308011602.3CC1C0244C@keescook/ [1]
Link: https://github.com/KSPP/linux/issues/90 [2]
Link: https://docs.kernel.org/process/deprecated.html#simple-strtol-simple-strtoll-simple-strtoul-simple-strtoull

 [3]
Cc: linux-hardening@vger.kernel.org
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230802-wifi-ipw2x00-refactor-v2-1-d33f765e9cd5@google.com