platform/x86: dell_rbu: avoid uninit value usage in packet_size_write()

platform/x86: dell_rbu: avoid uninit value usage in packet_size_write()

Ensure the temp value has been properly parsed from the user-provided
buffer and initialized to be used in later operations.  While at it,
prefer a convenient kstrtoul() helper.

Found by Linux Verification Center (linuxtesting.org) with Svace static
analysis tool.

Fixes: ad6ce87e5bd4 ("[PATCH] dell_rbu: changes in packet update mechanism")
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Link: https://patch.msgid.link/20260403134240.604837-1-pchelkin@ispras.ru
[ij: add include]
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>