smbus: Fix spd_data_generate() for number of banks > 2

spd_data_generate() splits @ram_size bytes into @nbanks RAM banks of
1 << sz_log2 MiB each, like this:

    size = ram_size >> 20; /* work in terms of megabytes */
    [...]
    nbanks = 1;
    while (sz_log2 > max_log2 && nbanks < 8) {
        sz_log2--;
        nbanks++;
    }

Each iteration halves the size of a bank, and increments the number of
banks.  Wrong: it should double the number of banks.

The bug goes back all the way to commit b296b664 "smbus: Add a
helper to generate SPD EEPROM data".

It can't bite because spd_data_generate()'s current users pass only
@ram_size that result in *zero* iterations:

    machine     RAM size    #banks  type    bank size
    fulong2e     256 MiB         1   DDR      256 MiB
    sam460ex    2048 MiB         1   DDR2    2048 MiB
                1024 MiB         1   DDR2    1024 MiB
                 512 MiB         1   DDR2     512 MiB
                 256 MiB         1   DDR2     256 MiB
                 128 MiB         1   SDR      128 MiB
                  64 MiB         1   SDR       64 MiB
                  32 MiB         1   SDR       32 MiB

Apply the obvious, minimal fix.  I admit I'm tempted to rip out the
unused (and obviously untested) feature instead, because YAGNI.

Note that this is not the final result, as spd_data_generate() next
increases #banks from 1 to 2 if possible.  This is done "to avoid a
bug in MIPS Malta firmware".  We don't even use this function with
machine type malta.  *Shrug*

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200422134815.1584-5-armbru@redhat.com>