isa: Implement the max_num_isa_dev macro

max_num_isa_dev is a macro to determine the maximum possible number of
ISA devices which may be registered in the I/O port address space given
the address extent of the ISA devices.

The highest base address possible for an ISA device is 0x3FF; this
results in 1024 possible base addresses. Dividing the number of possible
base addresses by the address extent taken by each device results in the
maximum number of devices on a system.

Signed-off-by: William Breathitt Gray <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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