Changes

bts-trx: Set max TRXD buf size to UINT16_MAX

Otherwise trying to allocate a msgb from it fails:
In file included from /git/osmo-bts/src/osmo-bts-trx/l1_if.h:9,
                 from /git/osmo-bts/src/osmo-bts-trx/trx_if.c:50:
/git/osmo-bts/src/osmo-bts-trx/trx_if.c: In function ‘trx_if_send_burst’:
/git/osmo-bts/src/osmo-bts-trx/trx_if.h:6:33: error: unsigned conversion from ‘int’ to ‘uint16_t’ {aka ‘short unsigned int’} changes value from
65536’ to ‘0’ [-Werror=overflow]
    6 | #define TRXD_MSG_BUF_SIZE       65536
      |                                 ^~~~~
/git/osmo-bts/src/osmo-bts-trx/trx_if.c:1126:48: note: in expansion of macro ‘TRXD_MSG_BUF_SIZE’
 1126 |                 trx_data_last_msg = msgb_alloc(TRXD_MSG_BUF_SIZE, "tx_trxd");
      |                                                ^~~~~~~~~~~~~~~~~

Change-Id: I980faea05dee405c974636f41ee723659d9c5229