Staging: rtl8187se: r8180_core.c: mark symbols as static

Fix sparse warnings for undeclared symbols not marked static like:
390:6: warning: symbol 'buffer_free' was not declared. Should it be static?
1031:5: warning: symbol 'ComputeTxTime' was not declared. Should it be static?

Signed-off-by: Anmol Sarma <me@anmolsarma.in>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>