net: dsa: sja1105: fix ptp link error

Due to a reversed dependency, it is possible to build
the lower ptp driver as a loadable module and the actual
driver using it as built-in, causing a link error:

drivers/net/dsa/sja1105/sja1105_spi.o: In function `sja1105_static_config_upload':
sja1105_spi.c:(.text+0x6f0): undefined reference to `sja1105_ptp_reset'
drivers/net/dsa/sja1105/sja1105_spi.o:(.data+0x2d4): undefined reference to `sja1105et_ptp_cmd'
drivers/net/dsa/sja1105/sja1105_spi.o:(.data+0x604): undefined reference to `sja1105pqrs_ptp_cmd'
drivers/net/dsa/sja1105/sja1105_main.o: In function `sja1105_remove':
sja1105_main.c:(.text+0x8d4): undefined reference to `sja1105_ptp_clock_unregister'
drivers/net/dsa/sja1105/sja1105_main.o: In function `sja1105_rxtstamp_work':
sja1105_main.c:(.text+0x964): undefined reference to `sja1105_tstamp_reconstruct'
drivers/net/dsa/sja1105/sja1105_main.o: In function `sja1105_setup':
sja1105_main.c:(.text+0xb7c): undefined reference to `sja1105_ptp_clock_register'
drivers/net/dsa/sja1105/sja1105_main.o: In function `sja1105_port_deferred_xmit':
sja1105_main.c:(.text+0x1fa0): undefined reference to `sja1105_ptpegr_ts_poll'
sja1105_main.c:(.text+0x1fc4): undefined reference to `sja1105_tstamp_reconstruct'
drivers/net/dsa/sja1105/sja1105_main.o:(.rodata+0x5b0): undefined reference to `sja1105_get_ts_info'

Change the Makefile logic to always build the ptp module
the same way as the rest. Another option would be to
just add it to the same module and remove the exports,
but I don't know if there was a good reason to keep them
separate.

Fixes: bb77f36a ("net: dsa: sja1105: Add support for the PTP clock")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>