eal/ppc: remove the braces in memory barrier macros

eal/ppc: remove the braces in memory barrier macros

Calling rte_smp_{w/r}mb macro expands into a compound block, which
would break compiling a else clause following it, if that calling
place has been terminated already with ";", as in below code.
This patch adds { } around this macro to allow compiling else too.

Fixes: d23a6bd04d ("eal/ppc: fix memory barrier for IBM POWER")
Fixes: 05c3fd7110 ("eal/ppc: atomic operations for IBM Power")
Cc: stable@dpdk.org
Signed-off-by: Gowrishankar Muthukrishnan <gowrishankar.m@linux.vnet.ibm.com>