rt2500usb: don't mark register accesses as inline

When CONFIG_KASAN is set, we get a rather large stack here:

drivers/net/wireless/ralink/rt2x00/rt2500usb.c: In function 'rt2500usb_set_device_state':
drivers/net/wireless/ralink/rt2x00/rt2500usb.c:1074:1: error: the frame size of 3032 bytes is larger than 100 bytes [-Werror=frame-larger-than=]

If we don't force those functions to be inline, the compiler can figure this
out better itself and not inline the functions when doing so would be harmful,
reducing the stack size to a merge 256 bytes.

Note that there is another problem that manifests in this driver, as a result
of the typecheck() macro causing even larger stack frames.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>