phy: mapphone-mdm6600: Fix write timeouts with shorter GPIO toggle interval

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>