staging: rtl8188eu: make efuse_power_switch() function static

staging: rtl8188eu: make efuse_power_switch() function static

The `rtl8188eu` driver is built as a kmod in order to avoid symbol
conflicts (at link-time) with other Realtek drivers.

Internally, we use this driver as builtin [vs kmod], and we've identified
the `efuse_power_switch()` symbol to be conflicting at link-time with the
one from the `rtlwifi` driver.

An alternative solution would have been to rename the function, but it
doesn't look like it's being used outside of this driver, so just make it
static.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20191016065834.12186-1-alexandru.ardelean@analog.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>