drm/mipi-dbi: fix a loop in debugfs code

This code will likely crash if we try to do a zero byte write.  The code
looks like this:

        /* strip trailing whitespace */
        for (i = count - 1; i > 0; i--)
                if (isspace(buf[i]))
			...

We're writing zero bytes so count = 0.  You would think that "count - 1"
would be negative one, but because "i" is unsigned it is a large
positive numer instead.  The "i > 0" condition is true and the "buf[i]"
access will be out of bounds.

The fix is to make "i" signed and now everything works as expected.  The
upper bound of "count" is capped in __kernel_write() at MAX_RW_COUNT so
we don't have to worry about it being higher than INT_MAX.

Fixes: 02dd95fe3169 ("drm/tinydrm: Add MIPI DBI support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[noralf: Adjust title]
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190821072456.GJ26957@mwanda

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