I've noticed that when writing data to the modem the writes can time out
at some point eventually. Looks like kicking the modem idle GPIO every
600 ms instead of once a second fixes the issue. Note that this rate is
different from our runtime PM autosuspend rate MDM6600_MODEM_IDLE_DELAY_MS
that we still want to keep at 1 second, so let's add a separate define for
PHY_MDM6600_IDLE_KICK_MS.

Fixes: f7f50b2a ("phy: mapphone-mdm6600: Add runtime PM support for n_gsm on USB suspend")
Cc: Marcel Partap <mpartap@gmx.net>
Cc: Merlijn Wajer <merlijn@wizzup.org>
Cc: Michael Scott <hashcode0f@gmail.com>
Cc: NeKit <nekit1000@gmail.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>