Porting Linux Drivers to RTOS
Porting the driver manually is the only way to go about it if you're unwilling to write the driver from scratch. @jeremy-kerr's right, you have license issues to be careful about, but essentially what this will mean if the driver is GPL is that your ported driver will also be GPL. You won't have to open up you're entire RTOS source code.

Three months is probably a conservative estimate for the porting effort; if you can identify the APIs used (network, interrupt, timers, waiting/locking, I/O, callback, file operation, etc.) and create a rough-and-ready map between the Linux API used and what is available on your RTOS it will make the process go MUCH more quickly, especially since you can then start to stub out the Linux API call used and "translate" it to the API and data structures used on your RTOS.

It won't be a simple task, but it's certainly doable and if the Linux driver is written well then you will have a very good idea of what each part does and why, which always makes porting easier. Good luck!