drivers: modem: gsm: Use Kconfig macros directly

Use the Kconfig macros of MODEM_GSM_RX_STACK_SIZE and
MODEM_GSM_WORKQ_STACK_SIZE directly without another macros.

They are well within the column limit and this make them
more obvious that they are actually configurable by Kconfig.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>