NET: sa11x0-ir: fix size of SIR transmit buffer

The SIR transmit buffer was being allocated as 4000 bytes.  IrDA now
has constants for the buffer sizes, and defines the maximum wrapped
SIR packet to be 4269 bytes as indicated by IRDA_SIR_MAX_FRAME.  Use
this definition to allocate the transmit buffer instead.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>