m68k: Fix unused variable warning in speed.c

The following warning was produced, fix it:

speed.c: In function 'get_clocks':
speed.c:94:15: warning: variable 'bPci' set but not used [-Wunused-but-set-variable]

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: "Jin Zhengxiong-R64188" <R64188@freescale.com>
Cc: Jason Jin <jason.jin@freescale.com>