staging: r8188eu: use mgmt to set the delba reason code

Use the mgmt struct to set the delba reason code.

Replace the numeric value with a define and remove the temporary variable.

Tested-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220602193726.280922-7-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>