staging: gpib: make global 'usec_diff' functions static

staging: gpib: make global 'usec_diff' functions static

Trying to build both gpib_bitbang and lpvo_usb_gpib into the kernel
reveals a function that should have been static and is also duplicated:

x86_64-linux-ld: drivers/staging/gpib/lpvo_usb_gpib/lpvo_usb_gpib.o: in function `usec_diff':
lpvo_usb_gpib.c:(.text+0x23c0): multiple definition of `usec_diff'; drivers/staging/gpib/gpio/gpib_bitbang.o:gpib_bitbang.c:(.text+0x2470): first defined here

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20241212154245.1411411-1-arnd@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>