video: sm501fb: Improve a size determination in sm501fb_probe()

Replace the specification of a data structure by a pointer dereference
as the parameter for the operator "sizeof" to make the corresponding size
determination a bit safer according to the Linux coding style convention.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Cc: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: Bhumika Goyal <bhumirks@gmail.com>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>