sbus: add missing MODULE_DESCRIPTION() macros

sbus: add missing MODULE_DESCRIPTION() macros

With ARCH=sparc, make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/sbus/char/envctrl.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/sbus/char/flash.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/sbus/char/uctrl.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/sbus/char/bbc.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: Andreas Larsson <andreas@gaisler.com>
Link: https://lore.kernel.org/r/20240619-md-sparc-drivers-sbus-char-v1-1-85acadb8f789@quicinc.com
Signed-off-by: Andreas Larsson <andreas@gaisler.com>