USB: usbserial: fix mos7720 dependencies

Fix mos7720 Kconfig dependencies.
When an enabled bool selects a tristate, the tristate becomes =y,
even if it should be limited to modular, so limit the bool kconfig
option to configs that will build cleanly.
Also change the if-block to a simple depends on.

drivers/built-in.o: In function `mos7720_release':
mos7720.c:(.text+0xad432): undefined reference to `parport_remove_port'
drivers/built-in.o: In function `mos7715_parport_init':
mos7720.c:(.text+0xae197): undefined reference to `parport_register_port'
mos7720.c:(.text+0xae210): undefined reference to `parport_announce_port'
drivers/built-in.o:(.data+0x201c8): undefined reference to `parport_ieee1284_read_nibble'
drivers/built-in.o:(.data+0x201d0): undefined reference to `parport_ieee1284_read_byte'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>