staging/easycap: don't mask return value of usb_control_msg() by 0xFF

masking return value of usb_control_msg() will mask negative
error values into positive.

Cc: Mike Thomas <rmthomas@sciolus.org>
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Reviewed-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>