staging/easycap: use regget for register back reading

Use regget to reading back what was written to a register.
This required changning size argument to regget signature

On the way remove usless variable casting

Cc: Mike Thomas <rmthomas@sciolus.org>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>