r92su: don't run "make all"

The "all" parameter in "make all" gets passed to the
kernel's main makefile. This causes errors like:

make -C /lib/modules/3.10.10-031010-generic/build
M=/home/media/rtl8192su/r92su CONFIG_R92SU=m CONFIG_R92SU_DEBUGFS=y
CONFIG_R92SU_WPC=y all EXTRA_CFLAGS="-DDEBUG -DCONFIG_R92SU=m
-DCONFIG_R92SU_DEBUGFS=y -DCONFIG_R92SU_WPC=y"
make[1]: Entering directory `/usr/src/linux-headers-3.10.10-031010-generic'
make[1]: *** No rule to make target `vmlinux', needed by `all'.  Stop.
make[1]: Leaving directory `/usr/src/linux-headers-3.10.10-031010-generic'
make: *** [all] Error 2

Reported-by: Brian O'Connor <bocon13@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>