m68k: atari - Rename "mfp" to "st_mfp"

http://kisskb.ellerman.id.au/kisskb/buildresult/72115/:
| net/mac80211/ieee80211_i.h:327: error: syntax error before 'volatile'
| net/mac80211/ieee80211_i.h:350: error: syntax error before '}' token
| net/mac80211/ieee80211_i.h:455: error: field 'sta' has incomplete type
| distcc[19430] ERROR: compile net/mac80211/main.c on sprygo/32 failed

This is caused by

| # define mfp ((*(volatile struct MFP*)MFP_BAS))

in arch/m68k/include/asm/atarihw.h, which conflicts with the new "mfp" enum in
net/mac80211/ieee80211_i.h.

Rename "mfp" to "st_mfp", as it's a way too generic name for a global #define.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>