fsi: core: Fix small accesses and unaligned offsets via sysfs

Subtracting the offset delta from four-byte alignment lead to wrapping
of the requested length where `count` is less than `off`. Generalise the
length handling to enable and optimise aligned access sizes for all
offset and size combinations. The new formula produces the following
results for given offset and count values:

    offset  count | length
    --------------+-------
    0       1     | 1
    0       2     | 2
    0       3     | 2
    0       4     | 4
    0       5     | 4
    1       1     | 1
    1       2     | 1
    1       3     | 1
    1       4     | 1
    1       5     | 1
    2       1     | 1
    2       2     | 2
    2       3     | 2
    2       4     | 2
    2       5     | 2
    3       1     | 1
    3       2     | 1
    3       3     | 1
    3       4     | 1
    3       5     | 1

We might need something like this for the cfam chardevs as well, for
example we don't currently implement any alignment restrictions /
handling in the hardware master driver.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20191108051945.7109-6-joel@jms.id.au


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>