fbdev: mbx: fix a misspelled variable name

A recent cleanup introduced a build failure when a variable
was spelled incorrectly:

In file included from drivers/video/fbdev/mbx/mbxfb.c:881:
drivers/video/fbdev/mbx/mbxdebugfs.c: In function 'mbxfb_debugfs_init':
drivers/video/fbdev/mbx/mbxdebugfs.c:217:2: error: 'mbfi' undeclared (first use in this function); did you mean 'mfbi'?
  mbfi->debugfs_dir = dir;
  ^~~~
  mfbi
drivers/video/fbdev/mbx/mbxdebugfs.c:217:2: note: each undeclared identifier is reported only once for each function it appears in
drivers/video/fbdev/mbx/mbxdebugfs.c:213:21: error: unused variable 'mfbi' [-Werror=unused-variable]
  struct mbxfb_info *mfbi = fbi->par;
                     ^~~~

Fixes: 72aed9e3

 ("fbdev: mbx: fix up debugfs file creation")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>