drivers: modem: gsm: Add context to DEVICE_INIT()
drivers: modem: gsm: Add context to DEVICE_INIT()

Instead of using global static variable "gsm" everywhere,
store the context when calling DEVICE_INIT(). Then in the device
init function get the context from the device struct. This way
it is possible to use the same functions for implementing two
gsm modem instances. Currently this is not fully possible because
the context is not passed via uart_pipe API and modem_cmd
callbacks. So some future work would still be needed.

In practice this commit does not change anything, it just makes
it a little bit easier to have two instance of this modem which
might be quite unlikely case. Anyway, the driver now follows the
same style how some other drivers are done like Ethernet etc.

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

Vudentz/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr