drivers: ieee802154: rf2xx: Rem trx_state variable

The rx timeout timer callback need update trx_state variable and this
variable is protected by a mutex. Because of that, when compiling the
system with CONFIG_ASSERT=y the system reports 'ASSERTION FAIL
[!arch_is_in_isr()] @ ZEPHYR_BASE/kernel/include/ksched.h:262'.

This refactor the driver remove trx_state variable dependency and
consequently removes phy_mutex and rx timeout timer to be compliant
with kernel rules.

Fixes: #23198

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>