firmware_loader: Replace simple_strtol() with kstrtoint()

firmware_loader: Replace simple_strtol() with kstrtoint()

Replace deprecated simple_strtol() calls with kstrtoint() in
timeout_store() and firmware_loading_store() functions to
improve input validation and error handling. The simple_strtol()
function does not provide proper error checking for invalid input,
while kstrtoint() returns an error for malformed strings.

This change adds proper validation for user input from sysfs attributes,
returning -EINVAL for invalid numeric strings instead of silently accepting
potentially malformed input. The behavior for valid numeric input remains
unchanged.

The simple_strtol() function is deprecated in favor of kstrtoint() family
functions which provide better error handling and are recommended for new
code and replacements.

Signed-off-by: Kaushlendra Kumar <kaushlendra.kumar@intel.com>
Link: https://patch.msgid.link/20250925063812.2269501-1-kaushlendra.kumar@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>