cli: account for additional digit for frequencies above 10GHz
When using 802.11ad, the frequency string will no longer fit in the
preallocated buffer of 10 bytes. In the best case this leads to the
'z' character being truncated in the CLI output:
Mode: Client Channel: 1 (58.320 GH)
Fix that by allocating a large enough buffer.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>