staging: r8188eu: use ARRAY_SIZE for mlme_sta_tbl

Use ARRAY_SIZE instead of hard-coding the number of entries in the
mlme_sta_tbl array.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220417102221.466524-7-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>