MIPS: PowerTV: Fix defconfigs for coverage builds

The toolchain prefix is most likely be site specific and is not
guaranteed to always be "mips-linux-gnu-", so simply don't specify one.
A quick "git grep" shows this to be consistent amongst other cross
compiled targets.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/configs/powertv_defconfig

@@ -6,7 +6,7 @@ CONFIG_HZ_1000=y
 CONFIG_PREEMPT=y
 # CONFIG_SECCOMP is not set
 CONFIG_EXPERIMENTAL=y
-CONFIG_CROSS_COMPILE="mips-linux-"
+CONFIG_CROSS_COMPILE=""
 # CONFIG_SWAP is not set
 CONFIG_SYSVIPC=y
 CONFIG_LOG_BUF_SHIFT=16