sfc: Fix maximum array sizes for various MCDI commands

The maximum array sizes have been calculated on the basis of a maximum
SDU size of 255 bytes, whereas the actual maximum is 252 bytes.
Constructing a larger SDU will result in a BUG_ON in efx_mcdi_copyin.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>