mx6slevk: Use the default CONFIG_SYS_PBSIZE

mx6slevk: Use the default CONFIG_SYS_PBSIZE

Entering the maximum number of characters defined by CONFIG_SYS_CBSIZE into
the console and hitting enter afterwards, causes a hang in the system because
CONFIG_SYS_PBSIZE is not capable of storing the extra characters of the error
message:
"Unknown command '' - try 'help'".

Use the default CONFIG_SYS_PBSIZE definition from config_fallbacks.h to solve
this problem.

Cc: Jason Liu <r64343@freescale.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Jason Liu <r64343@freescale.com>