wireless: Add channel/frequency conversions to ieee80211.h

Added mappings for FHSS, DSSS and OFDM channels - with macros to point
HR DSSS and ERP to the DSSS mappings. Currently just static inline
functions.

Use the new functions in the older fullmac drivers. This eliminates a
number of const static buffers and removes a couple of range checks that
are now redundant.

Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Richard Farina <sidhayn@gmail.com>
Acked-by: Jeroen Vreeken <pe1rxq@amsat.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>