staging: r8188eu: use ieee80211 define for fragment number

Use the IEEE80211_SCTL_FRAG define to extract the fragment number from an
incoming frame.

pattrib->frag_num must be in host endianness, we have to convert
hdr->seq_ctrl, this field is little-endian.

Remove the local GetFragNum macro, it is not used any more.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220327180944.712545-10-martin@kaiser.cx


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