wifi: mwifiex: Support SD8978 chipset

The Marvell SD8978 (aka NXP IW416) uses identical registers as SD8987,
so reuse the existing mwifiex_reg_sd8987 definition.

Note that mwifiex_reg_sd8977 and mwifiex_reg_sd8997 are likewise
identical, save for the fw_dump_ctrl register:  They define it as 0xf0
whereas mwifiex_reg_sd8987 defines it as 0xf9.  I've verified that
0xf9 is the correct value on SD8978.  NXP's out-of-tree driver uses
0xf9 for all of them, so there's a chance that 0xf0 is not correct
in the mwifiex_reg_sd8977 and mwifiex_reg_sd8997 definitions.  I cannot
test that for lack of hardware, hence am leaving it as is.

NXP has only released a firmware which runs Bluetooth over UART.
Perhaps Bluetooth over SDIO is unsupported by this chipset.
Consequently, only an "sdiouart" firmware image is referenced, not an
alternative "sdsd" image.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/536b4f17a72ca460ad1b07045757043fb0778988.1674827105.git.lukas@wunner.de