video: bfin_adv7393fb: Convert to kstrtouint_from_user

This patch removes a call to the deprecated simple_strtoul function
and simplifies the code by replacing two function calls with one
call to kstrtouint_from_user.

-Simplify the adv7393_write_proc function by replacing the
 simple_strtoul and copy_from_user calls with one call
 to kstrtouint_from_user.

-Change the count parameter from unsigned long to size_t as
 this is the type that the kstrtouint_from_user function expects.
 (size_t is what will be passed to the adv7393_write_proc function
 by the proc write handler function proc_file_write anyway)

Signed-off-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>