usb: typec: tcpm: Fix SINK_DISCOVERY current limit for Rp-default

usb: typec: tcpm: Fix SINK_DISCOVERY current limit for Rp-default

This is a regression introduced by 1373fefc6243 ("usb: typec: tcpm:
Allow slow charging loops to comply to pSnkStby")

When Source advertises Rp-default, tcpm would request 500mA when in
SINK_DISCOVERY, Type-C spec advises the sink to follow BC1.2 current
limits when Rp-default is advertised.
[12750.503381] Requesting mux state 1, usb-role 2, orientation 1
[12750.503837] state change SNK_ATTACHED -> SNK_STARTUP [rev3 NONE_AMS]
[12751.003891] state change SNK_STARTUP -> SNK_DISCOVERY
[12751.003900] Setting voltage/current limit 5000 mV 500 mA

This patch restores the behavior where the tcpm would request 0mA when
Rp-default is advertised by the source.
[   73.174252] Requesting mux state 1, usb-role 2, orientation 1
[   73.174749] state change SNK_ATTACHED -> SNK_STARTUP [rev3 NONE_AMS]
[   73.674800] state change SNK_STARTUP -> SNK_DISCOVERY
[   73.674808] Setting voltage/current limit 5000 mV 0 mA

During SNK_DISCOVERY, Cap the current limit to PD_P_SNK_STDBY_MW / 5 only
for slow_charger_loop case.

Fixes: 1373fefc6243 ("usb: typec: tcpm: Allow slow charging loops to comply to pSnkStby")
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Link: https://lore.kernel.org/r/20210510211756.3346954-1-badhri@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>