staging: rtl8187se: Deleted rtl8180_len2duration functions in r8180_core.c

After the next warning messages by sparse tools:

drivers/staging/rtl8187se/r8180_core.c:1621:5: warning: symbol 'rtl8180_len2duration' was not declared. Should it be static?
drivers/staging/rtl8187se/r8180_core.c:1621:12: warning: ‘rtl8180_len2duration’ defined but not used [-Wunused-function]
 static u16 rtl8180_len2duration(u32 len, short rate, short *ext)
            ^

And the next search:
$ grep -ir rtl8180_len2duration ./*
./r8180_core.c:u16 rtl8180_len2duration(u32 len, short rate, short *ext)
Binary file ./r8187se.ko matches
Binary file ./r8187se.o matches

I decided to delete rtl8180_len2duration function.

Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>