staging: gdm724x: Replace macro GDM_TTY_READY with static inline function

Replace the macro GDM_TTY_READY with a static inline function
to follow the Linux kernel coding style.

Signed-off-by: Brent Pappas <bpappas@pappasbrent.com>
Link: https://lore.kernel.org/r/20221117195443.19616-1-bpappas@pappasbrent.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>